Photons, on the other hand, have no color charge.
Quarks constantly change their color charge as they exchange gluons with other quarks.
These composites are color neutral. Now we understand that only those combinations are color neutral. Particles such as ud or dd that cannot be combined into color-neutral states are never observed experimentally. Color charge is always conserved. Therefore, when a quark emits or absorbs a gluon, that quark's color must change in order to conserve color charge.