Regarding mapped types, remember that { [K in T]: U } is a special form - it can't be combined with other properties within the { }. So there's not really a name for the [K in T: U] part because it's just part of the overall "mapped type" syntax { [K in T]: U }

Which... is confusing because Palette technically does have an index signature Palette is a mapped type, and mapped types don't have index signatures. The fact that both use [ ] is a syntactic coincidence.