Is your girlfriend taking a photochemistry class?

I suppose if you want to define "black" as "pure black" or "absolute black" then her professor may well be right. But I don't understand the statement about just painting a car red or blue because human eyes can't tell the difference. What difference? Between red, blue, and black? (No, I don't think that's what he meant, for the record) Or between different tones of black? Back in my days in the printing ink business we always dealt with color spectrometry for pigment matching, final color matching for critical work, etc. Color theory and the descriptors used can be confusing when you first start with this - is there too much black in that yellow, is that green too blue, is that a warm gray or cool gray, do you want a clean red or a dirty red, etc. A well trained eye can discern very fine color differences, just as a well trained palette can discern fine differences in taste. Then there's the complication of metamerism, whereby a color sample will appear to change under varying light conditions depending on the pigments used to create the color. This is more than the wavelength of ambient light altering tones on colors (think of a camera where the white balance is off and all colors look skewed), but it deals with how different pigments interact with different wavelengths of light. Two colors can match perfectly under light of a certain temperature, yet be several shades different when the light changes. The light is effecting the various pigments in different ways. It's not just the "visible color" - it goes way deeper than that. I suspect your girlfriend's prof is talking something along those lines.

But that doesn't mean you shouldn't paint a car black!