It's a good probability that at least one in 20 people can't see everything on your website. That's because a higher number of people than you probably expected are color blind in some way. Some statistics I've seen say that as many as 18 percent of people have some kind of visual limitation.
Around 90 percent of the people I tell this fact to are shocked - the rest know it because they're used to not seeing everything on a website. But what if they're missing something terribly important, like a news alert or a call for contributions or all your website navigation?
I love the Colorblind Web Page Filter because it makes it easy to see what your website might look like through a color-deficient eyes, those that can't see red/green or blue/yellow.
For some help choosing the right high-contrast colors for your site, try the Color Laboratory. It's handy because it "allows you to select colors and see how they appear next to one another, and in various foreground/background combinations. It also allows you to see those colors as they might appear to color-blind users."
Makes the incredibly important job of picking the right colors easier.