Combinations of red and blue make purple. But, there are no blends of red and green or blue and yellow. There are no reddish greens or bluish yellows and there are no names for these color mixtures. Red and green are opponent and so are blue and yellow. This is the first clue that some colors are processed differently and there are two separate pathways by which our visual systems process color.
Color vision is the ability to differentiate between colors.
In its simplest form, this is the ability to tell the difference between say red and green, though we have the ability to discriminate among many subtle shades. The fundamental basis of color vision is the presence of three photoreceptor types, each containing different but closely related pigments, which preferentially absorb three different wavelengths of light. We refer to these as red, green and blue cones. The signals generated by the three cone types are somehow sorted or combined by neuronal circuits in the retina. Among mammals, only primates have true, trichromatic color vision.