The force between nucleons in QCD.  The exchange of two colored gluons causes two quarks in each nucleon to change their colors (here blue changes to green and vice versa).  This process produces a force without violating the overall color neutrality of the nucleons.  The strength of the force depends on the separation of the different quark colors within each nucleon.

["Nuclear Physics, The Core of Matter, the Fuel of Stars", National Research Council, 1999]