RGB color charge, gluon exchange, confinement, and asymptotic freedom
Quantum Chromodynamics (QCD) is the theory of the strong nuclear force. Unlike electromagnetism, QCD has three types of charge — red, green, and blue — and its force carriers (gluons) themselves carry color charge. This leads to two remarkable properties: confinement and asymptotic freedom.
Just as electromagnetism has one charge (positive/negative), QCD has three: red, green, and blue. All observable particles must be "white" (color neutral) — either a color-anticolor pair (mesons) or all three colors (baryons).
Gluons carry one color and one anti-color charge. When a gluon is exchanged between quarks, the quarks change color. There are 8 types of gluons (not 9, because the color singlet combination is excluded).
Try to pull two quarks apart and the color field forms a "flux tube" — a string of gluon field energy. The energy stored grows linearly with distance. Eventually, there's enough energy to create a new quark-antiquark pair, and the string breaks into two hadrons. You can never isolate a single quark!
At high energies (short distances), the strong coupling constant α_s decreases — quarks behave almost as free particles. This is asymptotic freedom, discovered by Gross, Politzer, and Wilczek (2004 Nobel Prize). At low energies, α_s → 1 and perturbation theory breaks down: this is the confinement regime.
Only color-neutral combinations of quarks can exist as free particles. Build mesons (quark + antiquark) and baryons (three quarks) from the color palette.