Why do we perceive colour? (i.e., distinct values?)
"But what reason is there to not perceive light as we perceive sound, in a spectrum?"


We do perceive light in a spectrum. Not just in a spectrum, but in a combination of spectral components (because we rarely in nature observe true single-wavelength light). If you are thinking about names of colors, those are just words we made up to describe colors to other people, and the range and spacing of those colors differs across language groups. We do the same for sound in the context of music: B-flat versus an E, for example.


  Or, to go one step further, why did we need an extra type of cone in the middle, the yellow-green wavelength?


Colors are perceived in vision by the ratio of activity of different cones, and intensity and spectrum can be confused. If you have no "green" cones, there are big stretches of wavelengths that only activate one of the other two cones. In that situation, there is no way to discriminate between intensity and color.

If, on the other hand, you are wondering about transduction, it's because sound and light have different physical properties. The cochlea effectively "maps" sounds onto a physical arrangements of hair cells. You could do something similar with a prism, but you would lose spatial acuity (effectively, you would need a whole spectrum analyzer for the receptive field of each photoreceptor: your retina would have to be absolutely massive). Instead, color vision works by differential chemical sensitivity to different light wavelengths.

Your question doesn't seem to be much about evolution, besides the title, so I left off that part.