"Sufficient contrast" might seem to be a matter of taste or at least judgment, but the Web Content Accessibility Guidelines (WCAG 2.0) actually help us make a rational decision. These guidelines exist to help Web developers and designers make sure that people who have disabilities — in this case, colorblindness or other visual impairment — can understand their Web pages.

Fortunately, there are many tools available to help designers pick accessible color pairs. For two reasons, my favorite is Annika Hamann's Contrast-A.

First, if your first choice for a pair of colors does not have sufficient contrast, Contrast-A shows you how much you would need to adjust the hue, the saturation, or both of either color.

Second, unlike other tools, which let you analyze only one color pair at a time, Color-A lets you build a complete palette, making sure that each color pair that does need enough contrast actually has enough contrast. (For example, you might not care how much the background color of the main content contrasts with the background color of a sidebar. But you do want each text color to have enough contrast to its respective background. And you might want more contrast between the background color and the border color used in the "Submit Comment" button on this page.)

Also, like a number of other tools, Color-A lets you see how your color pairs look to people with four different kinds of colorblindness.

Best of all, although Annika accepts donations, you can get Contrast-A free at her website, contrast-a.dasplankton.com.

But let me add a word of caution here. Although WCAG 2.0 focuses on ensuring sufficient contrast, there are a few people for whom too much contrast is a problem. So achieving the highest contrast possible isn't necessarily a good idea. Just be sure that your important color pairs have enough contrast to conform to WCAG 2.0.