Alright. I know this is an old topic, but I stumbled upon it searching for a list of actual redheads in the game, and wanted to clear some things up:
-Aela is a redhead and a nord, the colour is poorly made in-game, but not non-existed.
-Ragnar the Red has his name most likely thanks to his hair colour (red). The changes of his skin being burned during all his travels is pretty small, and would have most likely ended up being mentioned in the story. Therefore, I would say it's very likely that his "red head" mentioned in the last sentence "And the braggart named Ragnar was boastful no more, when his ugly red head rolled around on the floor!" refers to having red hair. This also makes me thing having red hair is somewhat rare, but not impossible, which works well in with the idea nords are supposed to be Germanic in general, where red is the most common off all human "races", but still uncommon.
-Germanic people can have brown, even dark brown hair without being mixed race (just black hair is not really a thing), so it's very likely nords can as well. Saying nords are blond and grey (where grey is any race when they're old) is a bit shortsighted. And yes, Scottish are also Germanic. The fact there are redhead nords might even be considered "proof" for them representing all Germanic people, not just Nordic (who are also Germanic).
Other small mention
-TES races are based on real life human "races" but it shouldn't be taken too literally. Mers are based on (East-)Asians (beside maybe orcs), but for most, both skin and hair colours are very different. It's better to look at confirmed lore and in-game things (after checking they're not a misstake, like Olfina Gray-Mane being the daughter of Fralia and Eorlund, so she most likely shouldn't be greying yet (talking about hair colour). There are many mistakes like this found in creation kit, like people being said in lore to be family, but programming saying otherwise). If you want to make a real-world idea (like a viking) into a TES idea and have it lore-wise correct, you might need to make up some nice story sometimes, or make changes to your real-world idea.
And
ValenwoodCitizen, he's pretty wrong. Bosmer tend to have light brown till black-ish brown (but not natural black) hair. Dunmer on the other side are known to have red hair, and are the race where it is most common in the TES series.