All other perceived hues are similarly the result of the proportional activation of all three cone mechanisms by selective wavelengths of light. In an eye with normal vision the fovea contains three types of cones and no rods. The far periphery, near the ora serrata, contains only rods. The rest of the retina contains a mixture of rods and cones. Approaching the periphery of the retina the green cones disappear first, then the red cones, then the blue ones.
Color blindness is due to the absence or impairment, of one or two cone systems.
Dichromats are subjects with one cone system completely missing. They rely on the two remaining systems for rudimentary color vision. Monocrats are subjects who lack two of the three normal cone systems. A smaller number of people with color vision deficiency are dichromats , meaning that they have no sensitivity to either green or red. Tritanopia is the condition where there are no cones with cyanolabe.