Bribes are, well, just that. Bribes. Basically, you offer the colorist something shiny in exchange for kitsu, and they either decline or accept. Some colorists will accept items, some will not. Please check their individual workshops for this information. Generally the beginning price for a bribe is the stage you want + 50k. The harder you want the colorist to work, the more you should offer them.
Bribes are generally open at all times.
Bribes are generally open at all times.