Quarks and antiquarks also have a different kind of charge called "color", unrelated to electromagnetic charge. This comes from the strong interaction that holds the quarks together. As in all mesons, the color charges in a pion must be equal and opposite: "blue" with "anti-blue", "green" with "anti-green", or "red" with "anti-red". The effect of these color–anticolor pairings is that the pion's color charge is "colorless" (like a neutron is neutral). At the smallest distances, typically within an atomic nucleus, a small effect of the color charge remains and acts as the nuclear force that holds the nucleus together. Within the nucleus, then, "virtual" pions (and other "virtual" mesons) are exchanged between nucleons (protons and neutrons), pulling them together.