anticolor

If quarks carry color charge in three varieties (red, green, blue), then antiquarks — the antimatter partners of quarks — carry the opposite: anticolor, in three matching varieties called anti-red, anti-green, and anti-blue. Anticolor is to color what a minus sign is to a plus: it is exactly what you need to cancel the corresponding color back to nothing.

The cancelling is the whole point. A red quark and an anti-red antiquark, brought together, neutralize each other's color and form a colorless bundle — that is the recipe for a meson, a quark-antiquark pair. There is also a second, subtler way to make white: in the color scheme, a combination of all three anticolors (anti-red plus anti-green plus anti-blue) is itself colorless, which is why three antiquarks can bind into an antibaryon, the antimatter version of a proton or neutron.

Anticolor also matters for the force-carrier itself. A gluon, the particle that transmits the strong force, carries one unit of color and one unit of anticolor at the same time — for example, it might be red-antigreen. That mixed color charge is why gluons not only push quarks around but also pull on each other, a feature that sets the strong force dramatically apart from electromagnetism, whose photons carry no charge at all.