The whole point of boiling them is to remove the layer of skin and flesh between the horn and core, all horned animals need to have this removed or it will rot stink and eventually darken them as the stinking rotten ooze leaches out. Several factors such as how often the water is changed, how many critters are being boiled at once, duration of boiling, temp, etc will influence how much darkening the horns receive.
The best choice, which unfortunately does not meet import standards, is maceration. Wrapping the horns and skull in plastic or setting the skull in some warm water, just the skull not horns, and bagging the whole thing up then waiting for the horns to rot and popping them off and cleaning the horn sheaths as soon as they do then boiling the skull separately will preserve natural color and ideally should be done. This cannot be done to African trophies for a couple reasons, some horns such as black wildebeest cannot be removed from the core and the rotten mess would be trapped inside. The other reason and most important is that import requirements state that animal parts must be boiled, treated with gamma radiation salted dry with all flesh and fat removed or tanned, boiling is the only practical treatment for horns from these choices.
As a side note "boiling" horns to remove them is a lot harder on them than "simmering" them, far more collagen, glue holding them together, is removed. Unfortunately African trophies tend to have the hell boiled out of them, often to the point of damaging the horn.
Any way you look at it boiling horns is hard on them.
