The color portion of the Accessibility detail screen presents you with a color swatch of those colors used in the Web page text, in the left box, and those colors used in the fills & highlights on your page in the right box:
The reasons for presenting these color swatches are twofold:
1. Color contrast - To ensure that your page content is clearly visible to those with visitors with sight impairments it is a sensible idea to ensure clear contrast between text content and fills/highlights.
As an illustration of this point, consider the two examples below:
Clearly, as can be seen from the above example, the second set of text is far easier to read than the first. This may seem obvious, but it is surprising how many Web sites feature similarly poor choices of colors.
2. Color-blindness - This affects as much as 8% of the male population and 0.5% of the female population - so there is a high likelihood your site will be visited by someone affected by this common condition.
The text / fill comparison swatches allow you to quickly check for red/green text/fill combinations which might make the information difficult to read for someone with an affected form of color blindness.