You broadly categorize light into those colors. If you want to, though, you can focus on the in-between hues, such as reddish orange, orangish yellow, yellowish green, greenish blue, and bluish violet... and give these hues new names (just make them up). After that you can consider them to be the MAIN COLORS, while red, orange, yellow, etc. are the in-between ones. On the other hand, there's no reason to do that, since all you'd accomplish is putting yourself off the conventions that everyone else uses, without getting any redeeming social value out of your effort. But however you define color, you, and everyone, can agree that there is such a thing as color. We can understand that the physical difference between one color and another color is a fairly trivial difference in the frequency of the light. But, no matter how slight the frequency difference, the psychological importance of color is very profound. Color exists even thought the colors transition into each other in the spectrum, rather than being each neatly compartmentalized. To say "it's all ONE KIND of light" is a crass oversimplification. Also, color has a strong correlation with physical phenomena, such as the photoelectric effect, that has consequences independent of the human mind. A photon with an energy greater than the work function of an electron in an atom will unbind the electron and maybe send it clear of the material it was in. A photon with less than the required amount of energy will not do so. You know, this discussion sort of makes me think about racial differences in humans.