Quark Confinement: Quarks, which interact via the strong force, never exist singly. They are always bound either in quark anti-quark pairs called mesons, or in triplet combinations, like the proton and the neutron, called baryons.
If a pair of quarks is moving apart, the color force between them increases with distance.
Either the quarks are pulled back together, or the force lines snap creating a quark/anti-quark pair.
In this picture the force lines snap and color neutral mesons move apart
COSMOS to HUMANITY, Spring 2008
Lecture 2 , slide 24