A color that does not involve red, orange, yellow, green, blue, and/or purple, and it's combinations. Anybody ever see one?
All colors consist of pthalo blue-green (cyan), quinacridone rose (magenta), and hansa yellow (yellow). You cannot get a perfect black out of this combination, so in addition, a black component (represented as K) is added for most substances. These guys are known as the CMYK color model, and is why you don't have cadmium red, lemon yellow, and ultramarine blue in your printers.
Considering that humans can't see most of the light spectrum, there's billions upon billions of colors we can't see. Problem solved. Want a new color? Try gamma radiation.
Or you could become a butterfly. Or a mantis shrimp. °˖ ✧◝(○ ヮ ○)◜✧˖ °i mean, look at these bright little ****ers