the reason we perceive the spectrum as bands is primarily a function of our eyes and how they detect light at three main wavelengths. Whether the person considers the intermediate colors as unique or part of the dominant RGB bands, though, is generally socio-linguistic.

When you look carefully at a spectrum, there is no segregation between colors, the colors transition instead. We humans divide it into "bands" (unique colors) by choice, not because such bands are actually there. But ask someone where the line between colors actually and specifically happens, and I bet you would get lots of different answers.