http://racialreality.110mb.com/skincolor.html
I don't think any racial mixing has gone one with the Red Guard people, probably some though, I do notice some of them look mixed, like an Ethiopian persons face, more so in Skyrim then in Oblivion where they just looked like a standard African Negro (Negro means black, hope your people aren't sensitive race alarmists when race is spoken of, don't get all Hegelian dialect on me please Admins)
Another thing I notice is there are so many brown skinned Imperials, I think their long lost ancestry is in Hammerfell where they probably lived in small desert communities since the RedGuard lived in their massive cities and did not like outsiders, thats why so many Imperials must have brown skin. It is obvious that the Imperials are modeled after Romans but Brown skinned people were more prevalent during the Roman Kingdom days in Italy before the Italic tribes did significant mixing. During the Roman Empire which Cyrodiil obviously takes influence from, the people had mostly Olive-Fair skin with pale being very rare, light eyes being very rare, most people had brown-hazel eyes.
Mixing must go on between Bretons, Nords and Imperials, I have seen a lot of dark eyed Nords and Bretons. If you go by the real world influences the Lore takes from, Bretons would represent the Celtic tribes and Nords, Germanic tribes. Historically, Brown eyes and dark Brown eyes was virtually non existent, less then how many Romans had light eyes which was already low. I have seen some Nords and Bretons who have some very dark eyes. Mixing with the Imperials must have gone on a lot, they are arguably the most dominating race through the ages in Tamriel economically and even their Armies have dominated Tamriel.
So, my conclusion is all the Caucasian looking people mixed among each other and also took wives from Hammerfell which is rare but it has happened significantly in the past. Contrary to our real world where White women are trophies for Black men, I don't know what its like where you live but where I live in Canada, most women with Black men are ugly/fat and if shes good looking she is usually trashy and uneducated, used to go out everyday for about 2 years so I saw a lot of people.
Well I made most of this up to suit what I think about the Lore, is any of it right ? For those of you who know a lot about Elder Scrolls.