any photos? I'd suspect that one is not a self (black, blue, chocolate, lilac are selfs) Do they have pedigrees, they can hide the tort gene but not agouti which a gold is agouti and the tort gene combined.
A you know, Lilac is a dilute Chocolate. Chocolate vs Black happens at the B locus. The gene for Chocolate changes the shape of the eumelanin (brown/black pigment) granule, causing light to interact differently with it, so it appears brown rather than black. Gold is the non-extension gene, which is the E locus Non-extension doesn't care what shape the eumelanin granules are, it pushes them almost all of the way off the hairs, revealing the pheomelanin (yellow/ red pigment) that would otherwise have been covered up by the darker pigment. Non-extension is recessive, meaning that, in order for it to get expressed, the rabbit can't have a more dominant gene present at that locus. If a Lilac Buck and a Chocolate doe gave birth to non-extension babies, both the Lilac and the Chocolate must be carrying one copy of non-extension.