Just as electrically-charged particles interact by exchanging photons,
color-charged
particles exchange gluons in strong reactions.
The main difference between strong and electromagnetic interaction is the
fact that the strong force carrier particles themselves carry color charge.
Photons on the other
hand have no electric charge.
All particles with color charges are confined, that is, they cannot be found individually, but only in color-neutral composites (hadrons).
baryons
(three quark objects),
and mesons (quark-antiquark objects),
but
not,
for example,
four quark objects. Now we understand that only those combinations are
color neutral. Particles such as ud or dd that
cannot be combined into color-neutral states
are never observed experimentally.
Generations
Quark Interactions