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?