[Glenn E. Meyer]

If you are color blind - in the most common sense, you lack either the medium wavelength or long wavelength cone and are dichromatic. The reds and greens are indistinguishable on the dimension of red vs green perceptual hues.

If you are deficient then you have three cone receptors but the absorption spectrum of the three cones are abnornal. Usually one of the receptors has a peak too close to the other causing you to have limited discriminability compared to normal color vision.