Quarks are always found in combinations of two or three, never as isolated single particles. This behavior is explained by a concept known as color confinement, which is a fundamental aspect of the theory of quantum chromodynamics (QCD).
According to QCD, quarks carry a property called color charge, which is unrelated to the colors we perceive in everyday life. Color charge is a property associated with the strong nuclear force that binds quarks together. Quarks can have three different color charges: red, green, and blue, as well as their corresponding anticolors (anti-red, anti-green, and anti-blue).
The strong force between quarks is such that it becomes stronger as they move farther apart. As a result, it is energetically favorable for quarks to form bound states called hadrons, which include particles like protons, neutrons, and mesons. In the case of two quarks, they can combine to form a meson, whereas three quarks can combine to form a baryon.
When quarks combine to form a bound state, they do so in a way that ensures the resulting particle is colorless or "white." This means that the combination of quarks must have a net color charge of zero. For example, a proton consists of two up quarks (both with different colors) and one down quark (with a different color as well), resulting in a colorless combination.
Color confinement explains why we do not observe free individual quarks in isolation. As quarks are pulled apart, the strong force between them increases, until it becomes strong enough to create new quark-antiquark pairs or quark-antiquark-gluon combinations, preventing the isolation of individual quarks.
In summary, the requirement of color neutrality and the strong force between quarks lead to their combination in pairs or triplets, resulting in the observed composite particles in nature.