<P> Most placental mammals other than primates have only two types of color photoreceptor and are therefore dichromats, so it is possible that certain combinations of just two primaries might cover some significant gamut relative to the range of their color perception . Meanwhile, birds and marsupials have four color photoreceptors in their eyes, and hence are tetrachromats . There is one scholarly report of a functional human tetrachromat . </P> <P> The presence of photoreceptor cell types in an organism's eyes do not directly imply that they are being used to functionally perceive color . Measuring functional spectral discrimination in non-human animals is challenging due to the difficulty in performing psychophysical experiments on creatures with limited behavioral repertoires who cannot respond using language . Limitations in the discriminative ability of shrimp having twelve distinct color photoreceptors have demonstrated that having more cell types in itself need not always correlate with better functional color vision . </P> <P> The opponent process is a color theory that states that the human visual system interprets information about color by processing signals from cones and rods in an antagonistic manner . The theory states that every color can be described as a mix along the three axes of red vs. green, blue vs. yellow and white vs. black . The six colors from the pairs might be called "psychological primary colors", because any other color could be described in terms of some combination of these pairs . Although there is a great deal of evidence for opponency in the form of neural mechanisms, there is currently no clear mapping of the psychological primaries to neural substrates . </P> <P> The three axes of the psychological primaries were applied by Richard S. Hunter as the primaries for the colorspace ultimately known as CIELAB . The Natural Color System is also directly inspired by the psychological primaries . </P>

What is the significance of the primary colors