Quantum Chromodynamics
It turns out that fully consistency with the
hadrons requires that each quark type comes in
3 species which will symbolically be denoted as
red , blue and green .
Observed hadrons only correspond to
combinations that are white.
For example : proton is u(red), u(blue) and
d(green).
The analog of photons are gluons and they
come in 8 colours.