I think what we have here is a failure to understand how we see color, and how colors mix.
Primary colors exist because most of us have three different kinds of cone cells in our eyes, one whose peak sensitivity is to red, one to green, and one to blue.
Now a pure yellow light (say, the big yellow double-line in the emission spectrum for sodium, that makes sodium vapor streetlights so yellow) will excite both our red and green cone cells. By mixing red and green light, we can simulate that shade of yellow precisely enough that we can't tell the difference by looking at it, yet if we use a spectroscope, we can see that it looks completely different, and (depending on the sources of red and green light) might not contain any actual yellow light at all. That, incidentally, is how color television can work with only three colors. (If we ever meet a sentient race with different primaries, they're probably going to find our color television grossly unrealistic.)
With light, colors mixing is additive. Combining two colors gives a lighter color, and we can get something that looks like white light just by mixing red, green, and blue. But with dyes and pigments, it's subtractive: combining two colors produces something darker, and we have a different set of primaries: cyan (anti-red), magenta (anti-green) and yellow (anti-blue).
Now, let's imagine a batch of buttercream, made with all real butter, and natural vanilla extract. Since butter is yellow, and natural vanilla extract is brown, it's going to have a cream cast. Now, we can lighten that by whipping air into it, which increases the reflectance, but does nothing about the color cast: it's still going to be a warm white. Or we can neutralize the color cast by adding exactly the right amount of exactly the right shade in the blue-to-purple range, but that's going to darken it. Or, we can do both. It's still going to be, at best, a pale gray instead of a pure white, but if it's brilliantly lit, and surrounded by deep darkness, it's still going to look white, the same way the Moon (which is really a rather dark gray) looks like a brilliant, luminous white, when the sun hits it, in the deep dark of space.