Except we have seen the sub race? The game says the distinct subclans are the light skinned Veena, and the dark skinned Rava.
This is reflected in the in game footage we were shown, 2 Rava, and 2 Veena:
As well as the art:
The Viera on the far left could either be a Rava or a Veena, but is most likely a Veena to balance out 2 and 2 once again. There will certainly be some overlapping in skin tones, as this exists with plenty of the races we have currently such as these two comparisons of Duskwight and Wildwood Elezen, though I doubt you could tell which was which.
And some races like Lalafell don't even have any differences in their skin tone palettes at all. It's very likely Viera will have a gradient, where the palest options will be restricted to the Veena, and the darkest options restricted to the Rava, but the middle options will potentially be shared by both. The two Viera clan differences will come in the shape of other smaller things such as facial markings etc. Just like virtually every other race.