It is worth to remember, that summary (static) mass of u and d quarks in comparison with a proton or neutron mass (of which are compounds) is very small (1/100). Proton mass arise from relativistic effect of increase of the mass of fast moving quarks, but not too strong connected each other - income (of negative) potential energy is small. Quarks seems to be like small, sizzling grains of sand in centrifuge.