What few people seem to comprehend is that light is not visible. Some electromagnetic radiation is detectable and we refer to this range of detectable electromagnetic radiation as visible light. This is where most of the confusion arrises. Visible is a term used to describe conscious perception of visual representations. Trees, people and words on a page are visible.
When our eyes detect (not see) light, they send electrochemical impulses to our brains visual cortex. Here, our brain interprets these impulses and creates a visual representation of THE OBJECT, from which the light originates. We SEE the OBJECT (technically a visual representation of the object). So objects are visible, not light.
The color (visual sensation) of the object depends on the wavelength/wavelengths of light it emits/reflects, the cone/cones which detect it, the impluse/impulses sent to the visual cortex and finally, how our brain interprets the impulse/impulses. Different impulses/combinations of impulses, result in different visual sensations.
What I'm describing is how indirect realism affects our comprehension of colors. Purple is not in the visible light spectrum, but neither is red, orange, yellow or any other color. Colors are phenomenal sensations. They only exist in our brains.