My hypothesis:
In indicas, there are enzymes which convert THC to other cannabinoids. I hypothesise that the amber colour is generated by further processing of these cannabinoids. Pure sativas have much lower levels of these enzymes, this is what allows the THC to build to higher levels in these strains. In order for the amber colour to be produced, THC must be first be converted into other cannabinoids, which will not happen in a pure sativa due to lack of THC processing enzymes therefore sativas will not amber. That's my best guess.