the way i see it, all but the beast races (orcs, argonians, khajiit) and elves (aldmer, bosmer, dunmer) are just different colors of the same genetics basically. asking "are all redguards black" is like asking "are all white people white?" or "are all black people black?"
its just different colors really.
But this is a fantasy world, and genetics aren't going to behave or work the same way.
Notes on Racial Phylogeny already indicates that inheritance of features doesn't operate the same way as it does on Earth.
On Nirn, the "races" really do seem to be just that - at least, from gameplay mechanics. Different races have different starting attributes and different racial abilities. In-game lore presents the inhabitants of Nirn as falling within strict racial classifications - there are no 'half bosmer' or 'half orcs.'
As for Redguards, aren't they, lorewise, supposed to be of reddish hue (hence their name Ra'Gada)? :shrug: