What is fundamentally new in QCD is that the 8 gluons can interact mutually; they carry, like quarks, a colour charge. All "heavy" particles - the baryons - to which the proton and the neutron belong, consist of three quarks; a red, a green and a blue. It turns out that all "heavy" and "medium heavy" particles are colour neutral. The figure below shows how gluons are exchanged between the different quarks in a proton to glue them together.

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