Colour blindness is
Color blindness, a color vision deficiency, is the inability to perceive differences between some of the colors that others can distinguish.
according to wikipedia. Colour blindness is more in men ( close to about 10% of men have colour perception defect ), where as it is very rare in females.
Online Eye-sight test to check colour-vision deficiency:
Here are some tests that will help you clarify any doubts you may have if you suspect you are colour blind.
If you wonder what being colour-blind is like then Vischeck will show you how things look like to some one who is color blind.
Vischeck have a photoshop plug-in which you can download and see what images will look like. They also an online tool which serves the same purpose.
Some examples of how pictures may look like for colour blind people:
Important Webmaster resources while building websites for the colour blind :
If you are curious to know what your website looks like to colour blind visitors, then http://newmanservices.com/colorblind/default.asp will help you analyze it.
Webmasters can also make use of Colour Blindness Simulator, which helps you analyze how the images on your websites will be presented to your visitors.
Some more interesting links :
How do things look like for colour blind people http://webexhibits.org/causesofcolor/2.html