harlequin and tri only. in standard rex at least, neither harlequin nor tri are accepted for showing. if you breed them to another colour you can end up with the harlequined version of that colour, which is a DQ. it can also pop up down along the lineage too, so generally people won't want to buy a castor with harlequin in the pedigree, since they could possibly end up with unshowable rabbits, so keep that in mind if you want to sell any!
if they're mini rex, tri is accepted, but the rule about not crossing colours still stands.