Color Vision

What is color blindness?

Color vision is pretty complicated. If you're not color-deficient or color-blind then it's hard to understand what color-blind people see. There are four kinds of color vision:

Regular vision is Trichromatic - it uses all three colour receptors (red/green/blue). In practice, the cone cells in your eyes are called L, M and S (for long, medium and short wavelength reception), but the colors they 'see' are closer to Yellow, Green and Blue. The wavelengths they pick up are vastly overlapping, so green light hits all three in varying degrees (and all wavelengths hit your rod cells).

Anomalous Trichromat
People with Anomalous Trichromatic vision use all three color receptors but reception of one pigment is misaligned.
Protanomaly: reduced red sensitivity.
Deuteranomaly: reduced green sensitivity.
Tritanomaly: reduced blue sensitivity.

People with Dichromatic vision use only 2 of the 3 visual pigments - red, green or blue is missing.
Protanopia: unable to receive red.
Deuteranopia: unable to receive green.
Tritanopia: unable to receive blue.

Monochromat (Achromatopsia)
People with Monochromatic vision can only see one color, so their vision contains no 'color'.
Typical Monochromatic: unable to combine colors. Fully grayscale. Also known as Rod Monochromat.
Atypical Monochromatic: very low color recognition. Also known as Cone Monochromat.

Note: You might also like to do a bit of research into tetrachromats, who have four color pigments instead of the usual three!

Wikipedia has plenty of useful information about color blindness.