friend said:
All quarks have a color charge, right? And gluons have a color and anti-color charge, right? Quarks change their color due to interactions with gluons, right?
So one of the big problems with the way popular science discusses QCD is the way it describes color charges. In particular, it likes to say that the "white" meson can be made up of either red*antired, green*antigreen, or blue*antiblue in quark-antiquark pairs. This is even how I learned it in an advanced undergraduate quantum mechanics class. But a little thought already shows this is weird - shouldn't every meson then be tripled, since we have three different ways of combining the quark and antiquark to get the "white" meson?
Later, when I took quantum field theory and actually learned QCD mathematically, I found that a lot of what was said even in that advanced undergraduate course was just an inaccurate representation of what is actually mathematically going on. In particular, the nature of how non-abelian charges add up to become neutral ("white") is much more complicated than what is said using the "color" language.
So let's say each quark and antiquark comes in three possible colors and anticolors respectively - this accurately represents the math. (As the more advanced readers know, this is because quarks and antiquarks are in the fundamental and antifundamental representations of SU(3) respectively.) Now, how can we combine a quark and an antiquark to get a "color-neutral" object? If you learn the math behind QCD, you learn that this is a problem in group theory: the technical term is that you need to decompose tensor product of the representations of the quark and antiquark into direct sums, and then the allowed "white" states are any parts of that direct sum which are singlets (are trivial under SU(3)).
(If you have taken an undergraduate quantum mechanics course, you were actually doing part of this process with a different group when you learned the addition of angular momentum and Clebsch-Gordan coefficients.)
Now, if I call the three indices of the quark red, blue, and green, and the three indices of the antiquark antired, antiblue, and antigreen, then the mathematical procedure I mentioned above would actually tell me that there is only
one white state, and it is given by the superposition (red*antired + blue*antiblue + green*antigreen).
This is before getting to baryons. The popsci version says you combine red*green*blue=white. But the actual decomposition is some massively ugly group theory computation which I would need to brush up on my math skills to do. The problem is that the color "white" is actually referring to something called an "irreducible representation of a Lie algebra" (specifically white= "the trivial representation"), and the way in which representations combine to give you "white" cannot be described using some nice simple rule about mixing colors.
So I guess this whole post is to caution you against taking what you hear about color charge in pop sci (or even at some more advanced levels) at face value. It can lead you astray.