Hadrons are particles like the nucleons, the constituents of atoms of normal matter, or the mesons (the particles that nucleons exchange while interacting in the nucleus of an atom).
The strong interaction is carried by other particles, called gluons, which themselves have colour, and interact between themselves also. This is why hadrons emerge as a non-perturbative result of the strong interaction between quarks, given by Quantum Chromodynamics, QCD.
QCD assigns to quarks a given charge for the strong interaction, which is called colour. Quarks can have three strong charges or colours. But all hadrons are neutral : they have no net colour.
Intensive numerical simulations are needed to describe the highly non-perturbative mechanisms that generate hadrons. These simulations are implemented by discretizing space-time into a grid of points, and by finding the trajectories minimizing the energy.
Extraordinary predictions as the existence of an effective potencial between quarks at large distances, and the mass spectrum are obtained by these calculations.