What would our color spectrum look like if we had four (or two!) cones rather than three? I had already asked this question on asksciencefiction, but was told it probably belonged here.

FWIW, I'm asking mostly from a writing perspective, as in I want to have a species that varies in number of cones, or the specific wavelengths of cones than humans, but I'm having trouble wrapping my head around the idea. I'm currently learning about visual sensation and perception in one of my courses this semester, if that helps at all with answers either