BODY Colors

The WAI guidelines (4.2) state:

Use foreground and background color combinations that provide sufficient contrast when viewed by someone with color deficits or when viewed on a black and white screen.

In our A-Prompt program, we check the BODY element for background/text colors and if they are set we warn the user. But some color combinations are OK (white background, black text) and we shouldn't warn the user about them.

We would like to make the program smart enough so that if it sees a large contrast between the background and text colors, it will NOT warn the user. We're looking for ideas on how this could be done. What makes a good contrast? Just brightness? Just color difference? What about link colors?

Chris