One of my best friends has a color vision deficiency, or "color blindness," as it is also known. Being as that we are best of friends, I have asked him on several occasions to describe what he sees as I compare it to what I see when looking at things like posters, movies, the sky, and other colorful things so I could get a sense of how our vision was different. He's also the first to poke fun at himself when he shows up somewhere in purple pants and an orange sweater, or something as equally hilarious as that. "I had no idea, because, you know, I CAN'T SEE THOSE COLORS!"
The condition my friend has is called Protanopia, or basically a complete lack of red photoreceptors in the retina. Red hues appear black or dark; it is hereditary, sex-linked, and present in 1% of all males, according to Wikipedia's article on color blindness. Protanopia is a form of Dichromatism – this is what occurs when one of the cone pigments of the retina, and color vision is reduced to two dimensions.
I've often asked him if it's weird not being able to see a certain spectrum of colors, to which he often responds with "I have no idea, I've never been able to see that color, so how would I know?" I always feel pretty ridiculous after I ask him that.
One of the most important aspects to know about color blindness is that people with color vision impairment don't "confuse" colors or swap them in their mind – like "red is green," "blue is yellow," etc. Most people exhibiting color vision impairment learn to tell colors apart by their placement or location (like a traffic lights) or textures (like camouflage).
Look at the differences in colors as seen by people with differing forms of color vision impairment:
As seen by a person with normal color vision:
The same flag, as seen by someone with Protanopia (no red receptors):
Same flag again, but this time seen by someone with Deuteranopia (no green receptors):
Same flag once more, this time as seen by someone with Tritanopia (no blue receptors). Tritanopiais very rare:
The website Vischeck (http://www.vischeck.com/) can simulate color deficiencies on images that you upload- it's an interesting foray into color vision deficiency. Check it out.