This tool takes all of the references to colour from your web page including your CSS and nicely compares the background colours to the foreground colours. The report that gets tabulated shows very bluntly what instance is acceptable and what instance is not. The system analyses three things: Contrast Ratio, Brightness difference, and Color difference. By clicking on the rows, it also allows the user to find another colour that would replace the current one. Unfortunately, this feature is a little hard to click on.

The site can give the user an idea of what is lacking in terms of colour accessibility, but does not offer a comprehensive understanding of what the guidelines are. Explaining what the categories and the numbers mean might clarify the systems results.

Let me demystify it for you.

Contrast Ratio: Contrast is measured using a formula that gives a ratio ranging from 1:1 (no contrast) to 21:1 (maximum contrast). AA and AAA refers to the level of priority. Level AA are strongly recommended (mininum contrast is 4.5:1) to allow most users accessibility. Level AAA are suggested (mininum contrast is 7:1) to allow some users accessibility.

Algorithm for luminosity contrast ratio in Web Content Accessibility Guidelines 2.0 Glossary.

Brightness difference: Another formula that evaluates the difference between the background brightness, and the foreground brightness. This should be greater than 125.

Color difference: This formula refers to the difference between the background colour and the foreground colour  and should be greater than 500.

Formulas are explained in the document Techniques For Accessibility Evaluation And Repair Tools.

The limitation of these types of applications is that we cannot check the colour balance from images.  This is one of the reasons that it is not recommended to make any text into images. They cannot be viewed by external reading programs.

Check it out at: http://www.colorsontheweb.com/colorcontrast.asp