Both
quarks and
gluons carry a type of charge called color. Like
electric charge,
color charge is always conserved. But unlike the
electric charge, the
color charge comes in six varieties, three colors and three anti-colors. The colors are usually called
red,
green, and
blue. The idea is that we know that
protons and
neutrons as well as
hadrons are made up of
quarks. Yet we never see
color charge even if we try to break up
protons and
neutrons into their
constituent parts (colored quarks). So the objects that we observe, and therefore construct, must be
colorless or color neutral; which is why we cannot see individual
quarks. When each
quark in a
hadron has a different color:
red+ green + blue = white
the result is a color neutral object. This also allows the quark theory to describe another class of particles: mesons which have a quark and an anti-quark
color + anti-color = white
Gluons carry color/anti-color pairs that do not have to be the same color. There are 8 gluons as they each have one of the eight possible color/anti-color combinations.
See Quantum Chromodynamics