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Tools - Colour Contrast Analyser 1.1

This software has been superceded

A new Contrast Analyser is now available

Italian version: Versione italiana: Contrast Analyser - Versione 2.0

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What is the Colour Contrast Analyser?

It is primarily a tool for checking foreground & background colour combinations to determine if they provide good colour visibility. It also contains functionality to create simulations of certain visual conditions such as colour blindness.

Determining "colour visibility" is based on two sets of algorithms: Luminosity Contrast Ratio and Colour Difference and Brightness Difference, suggested by the World Wide Web Consortium (W3C):

Note: It's important to remember that neither the luminosity contrast ratio or the colour combination algorithm suggested by AERT have ever been recommendations; they're suggestions by two WAI working groups to help determine whether or not the contrast between two colours can be read by people with colour blindness or other visual impairments.

Luminosity Contrast Ratio

Guideline 1.4 of WCAG 2 (draft) states, make it easy to distinguish foreground information from background images or sounds.

The luminosity contrast ratio algorithm will become a recommendation when WCAG 2 becomes a recommendation if it is still listed in the guidelines. The algorithm was developed by Gregg Vanderheiden, Dave Kelso, and Aries Arditi at the Trace R&D Center.

(L1+.05) / (L2+.05) where L is luminosity and is defined as .2126*R + .7152*G + .0722B using linearised R, G, and B values. Linearised R (for example) = (R/FS)^2.2 where FS is full scale value (255 for 8 bit color channels). L1 is the higher value (of text or background) and L2 is the lower value.

The Gamma correction and RGB constants are derived from the Standard Default Color Space for the Internet (sRGB) , and the 0.05 offset is included to compensate for contrast ratios that occur when a value is at or near zero, and for ambient light effects.

The Gamma correction and RGB constants are derived from the Standard Default Color Space for the Internet (sRGB) , and the 0.05 offset is included to compensate for contrast ratios that occur when a value is at or near zero, and for ambient light effects.

Text or diagrams and their background must have a luminosity contrast ratio of at least 5:1 for level 2 conformance to guideline 1.4, and text or diagrams and their background must have a luminosity contrast ratio of at least 10:1 for level 3 conformance to guideline 1.4.

Colour Difference and Brightness Difference

This algorithm was suggested in the 26th of April 2000 working draft for Accessibility Evaluation and Repair Tools (AERT).

"Two colours are considered to provide good colour visibility if the brightness difference and the colour difference between the two colours are greater than a set range." The range suggested by the W3C is > 125 for colour brightness and > 500 for colour difference. Note: Hewlett Packard (HP) provide an online colour contrast verification tool that uses the W3C algorithms, but sets the Colour Difference range at > 400, which results in a greater range of foreground & background colour combinations being deemed acceptable.

Colour Brightness Formula

colour brightness is determined by the following formula:
((Red value X 299) + (Green value X 587) + (Blue value X 114)) / 1000 The difference between the background brightness, and the foreground brightness should be greater than 125.

Note: This algorithm is taken from a formula for converting RGB values to YIQ values. This brightness value gives a perceived brightness for a colour.

Colour Difference Formula

colour difference is determined by the following formula:
(maximum (Red value 1, Red value 2) - minimum (Red value 1, Red value 2)) + (maximum (Green value 1, Green value 2) - minimum (Green value 1, Green value 2)) + (maximum (Blue value 1, Blue value 2) - minimum (Blue value 1, Blue value 2)) The difference between the background colour and the foreground colour should be greater than 500.



The Colour Contrast Analyser (CCA) is useful to help determine, in particular, the legibility of text on a web page and the legibility of image based representations of text.

Assessing conformance with Checkpoint 2.2 of the Web Content Accessibility Guidelines 1.0

Checkpoint 2.2 of the WCAG 1.0 requires that

"foreground and background colour combinations provide sufficient contrast when viewed by someone having colour deficits, or when viewed on a black and white screen."

Use the CCA to test colour combinations against the W3C's suggested algorithms for determining "sufficient contrast".


Using the CCA

The user interface consists of 5 sections
  1. Colour Choice
  2. Results
  3. Colour Blindness Results
  4. Simulations
    • Select Window (list)
    • Select Image File
    • Simulation Viewer
  5. Options & Help Menus

1. Colour Choice

The Colour Choice controls are divided into 2 sections (foreground & background) each has 4 ways to choose a colour to test.

Interface para escolha da cor

  1. Colour pallet(s) accessed via a select box. Use this to help you test and choose suitable colour combinations.

    Paleta de cores

    The bottom (smaller) colour pallet consists of 10 shades of the currently selected base colour, located top-left, at varying degrees of opacity. The top row emulates opacity over a white background, the bottom over black . The opacity values are 100% opaque, 75%, 50%, 25% and 10% on the top row. The bottom row begins at 85% rather than 100% and continues on as the first.

    This pallet is useful for testing lighter and darker shades of the base colour, which may improve the visibility of the colour combination being tested.

    Note: This feature was inspired by the Color Palette Creator.

  2. Input of a colour value (in Hexadecimal or RGB format) into a text box. Useful if you have access to the Hex or RGB values of colours you want to check.

    Note: The colour value format can be changed via the "options > displayed colour value" menu

    hex value input box RGB input boxes

  3. Sampling of a colour from a web page (or anywhere else on the screen) using a magnifying colour sampler. colour sampler button

    Useful for quickly sampling, checking and obtaining the hex values of colours. Once the colour sampler button is selected a magnified area will appear around the cursor. The colour of the content under the tip of the cursor will appear in the colour pallet select box

    Once a colour is chosen with the colour sampler (left mouse click or 'enter' key) its hex value will appear in the 'hex value' text box and the results section will be updated to reflect the brightness and colour differences for the currently selected colour combination.

    Note: For fine control of the cursor use the arrow keys.

    Example of the magnified area around the cursor that occurs when the colour sampler is activated. The colour under the cursor tip appears in the colour pallet select box

  4. Red/Green/Blue slider controls

    red green and blue colour slider controls

    The slider controls were inspired by the colour contrast check at Thanks to pixeldiva for suggesting their inclusion

2. Results

The results section is divided into pass/fail icon & sample text results and full results: A pass is indicated by a Tick. A fail is indicated by a cross. A text box containing text in the foreground colour tested and a background using the background colour tested

pass/fail icon and sample text

The 'Full Results' displays the the colour values tested, result values for the test and what the pass/fail criteria are and indicates whether the contrast was found to be sufficient/insufficient.

Note: The results can be copied to the clipboard by activating the "copy results" button copy results button

results of the contrast check

3. Colour Blindness Results

Selecting the 'also show contrast results for colour blindness' check box will display summary results for 'normal vision' and 3 types of colour blindness (Protanopia, Deuteranopia, Tritanopia).

example colour blindness results

Note: The method of determining the colour value transformations for the 3 types of colour deficiency is from an algorithm described by Hans Brettel , Francoise Vienot and John Mollon in a paper which appeared in the Journal of the Optical Society of America V14, #10 pp2647.

Acknowledgements to the kind people at Vischeck who allowed us access to the source code of their implementation of the Brettel algorithm.


4. Options, Simulations & Help Menus


Selecting the 'always on top' menu item will keep the CCA window on top of all other windows that are open. Selecting 'font' menu item opens a dialog box that allows you to modify the font used for the text labels and content of the CCA. selecting the 'displayed colour values' allows a choice between Hex and RGB. Selecting 'algorithm' allows a choice between testing against the luminosity and colour/brightness algorithms.

Note: selecting some font types or sizes can result in clipping of text content.

Options menu


Documentation yet to be written....


Selecting the "help" Menu item Help menu will open the CCA documentation web page in a browser window.

Note: you will need to be connected to the internet to access the documentation.


Use via Keyboard

Colour Contrast Analyser Access keys
move through all controlstab key
open foreground colour pallet(s)F9
open background colour pallets(s)F10
move through colour palletsarrow keys or the tab key
activate foreground colour samplerF11
activate background colour samplerF12
move colour samplerarrow keys
choose colour in sampler or palletEnter
move red, green, blue slider controlsarrow keys
always on topctrl+S
displayed colour value - hexctrl+E
displayed colour value - RGBctrl+R
algorithm - luminosityctrl+L
algorithm - brightness/differencectrl+O
simulations - select window listctrl+I


Terms of Use Notice:

The Colour Contrast Analyser is Freeware.

By accessing or using the Colour Contrast Analyser, you acknowledge that you have read, understood and agree to be bound by the Colour Contrast Analyser Terms of Use.


This work is licensed under a Creative Commons License Creative Commons Licence.


Download and Installation Instructions

Please Note: The Colour Contrast Analyser does not contain any adware or spyware.

System Requirements

  • Microsoft Windows - Tested on Windows 2000 & XP

This software has been superceded

A new Contrast Analyser is now available



Installation instructions

Download, unzip and place the CCA folder where you like. Use the application by activating the Colour_Contrast_Analyser.exe file.



The Colour Contrast Analyser was developed by JUn of Wrong HTML Wrong Html Logo in collaboration with Steve Faulkner of Vision Australia.

It's development was inspired by the Juicy Studio Colour Contrast Analyser, the HP colour contrast verification tool and the Color Visibility Test Program by Chris Ridpath from the University of Toronto

Special thanks to Gez Lemon of Juicy Studio for the use of content from his site and his feedback during the development of the CCA. Also many thanks Andrew Arch, Sofia Celic and pixeldiva for their comments and suggestions.

To provide feedback on the Colour Contrast Analyser

Email Steve Faulkner []


Support WAT-C tool development

Make a using PayPal.


Trenton Moss from Webcredible: Using the Web Accessibility Toolbar

Steven Faulkner from TPG: Using the Web Accessibility Toolbar

Tool Internationalisation

If you're interested in contributing to the translation and localisation of our tools, please contact: Steve Faulkner

Tool Customisation

We develop customised versions of our tools to suit the web accessibility testing requirements of Government, Educational, Corporate and Not for Profit organisations.
Contact: Steve Faulkner

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