Upload any image and Stanford’s Vischeck program will show you how a color-blind person would see it.

The program lets you choose among three flavors of color blindness. This macaw appears as a protanope would see it, someone who can’t distinguish between colors in the green-yellow-red section of the spectrum.

About 10 percent of American men have some deficiency in color perception, but it’s not always a handicap. In some situations it’s actually an advantage: Color-blind hunters are unusually good at picking out prey against a confusing background, and color-blind soldiers can sometimes “see through” camouflage that fools everyone else.

In fact, it’s possible that in extreme situations we’re all color-blind. Some people claim that in extreme emergencies, like a train or aircraft crash, the brain’s visual system suspends color processing and switches to black and white. If that’s true, then designers should pay even more attention to the color of emergency brake handles, phones, etc.

If you’re interested, the Stanford page can also display your Web page as the color-blind would see it, and it even offers free PhotoShop plugins so you can experiment further.