This means they have no net color charge. This is the combination that makes a baryon . A state of three antiquarks, one of each anti-color, is also color neutral. This is an antibaryon. This too is a color-neutral combination.
Gluons have one color and one anti-color.
There are, however, only eight types of gluons, not the nine you might expect. That is, the combination that has an equal probability of containing each color together with its own anti-color. Thus there is no gluon with this color combination. This apparently arbitrary statement is part of the mathematics of color charge. All observed particles are color-neutral objects.