Color-charged particles cannot be found individually. For this reason, the
color-charged quarks are confined in groups (hadrons) with other
quarks. These composites are color neutral.
The development of the Standard Model's theory of the strong
interactions reflected evidence that quarks combine only into
baryons (three quark objects), and mesons (quark-antiquark objects),
but not, for example, four-quark objects. Now we understand that only
baryons (three different colors) and mesons (color and anticolor) are color-neutral.
Particles such as ud or uddd that
cannot be combined into color-neutral states are never observed.
The quarks in a given hadron madly exchange gluons. For this reason,
physicists talk about the color-force field which consists of the
gluons holding the bunch of quarks together.
If one of the quarks in a
given hadron is pulled away from its neighbors, the
color-force field "stretches" between that
quark and its neighbors. In so doing,
more and more energy is added to the color-force field as the quarks are
pulled apart. At some point, it is energetically
cheaper for the color-force field
to "snap" into a new quark-antiquark pair. In so doing, energy is conserved because the
energy of the color-force field is
converted into the mass of the new quarks,
and the color-force field can "relax" back to an unstretched state.
Quarks cannot exist individually because the color force increases as they are pulled apart.