I really wish I could help you out, but as far as I have seen, there is very little description of Loki’s physical appearance. Snorri Sturluson explicitly states that Loki is handsome, but that’s about it, and it’s unclear from where such a statement would have originated. Loki fights only with words and cunning, so I think it is a valid assumption that he is of a leaner build than those hardened by battle.
Speaking of hair though… Thor definitely has red hair, as that’s one of his kennings, but Loki’s hair colour, as far as I know, is not mentioned.
I imagine the pagans consider Loki a redhead because of his connection to fire, which I guess is a valid decision. However, Thor was specifically singled out as the red-haired god. If Loki was also red-haired, it wouldn’t make sense for Thor to be distinguished with such a nickname. It would get a bit confusing, especially since Loki and Thor always hung out together, and quite possibly lived together when in Asgard. I’m a bit hesitant to think of Loki as a red-head for this reason, but it’s as valid an assumption as any, really.
When Loki turned into a mare, he was a white mare, so perhaps we could take this to mean he is of fair hair or complexion? Shape shifting rules aren’t so clear-cut, but it’s a thought. Heimdallr was known for being the whitest god, and a great amount of emphasis was placed on Baldr’s fairness (of complexion, not behaviour), so would it not have also been mentioned, if Loki was among the fair Aesir? It’s difficult to say.
There is also something to be said for depicting Loki with pale skin and dark hair. I know nothing of horse breeding, but Loki and Svadilfari were both white horses, but their offspring Sleipnir somehow came out grey. That’s definitely clutching at straws, but I’m just tossing out ideas. Loki is also associated often with the Svartalfar,who are known for being swarthy (of dark complexion). Loki isn’t a Svartalf, so the association may amount to nothing, but the fire Jotnar seemed to be swarthy as well i.e. Surtr. Loki allegedly had a connection to fire, so this could be viable; however, Loki seems to have been of hrimthursar (Rime Jotnar) ancestry, so again, the details don’t exactly line up.
Well, I think Loki’s physical appearance is a bit of a mystery. Loki spent all his time doing such crazy things that no one noticed his hair colour or length – they only described him by his actions and posterity. Or, since no attention is called to Loki’s hair, maybe it was simply brown (which, although a beautiful colour for hair, does not stand out as much as others).
For the most part, I think a lot of these myths are intended to be filled in with our own imagination. They were passed down via oral tradition, and changed a lot over time, so Loki may have never even had a consistently decided hair length/colour.
I took a while to respond to this, because I really felt bad that I didn’t have a good answer for you. But after poring over all my my notes and texts for a few days, this is the best I can give you. I will keep looking, and I will definitely post a description if I make any discoveries in that vein. That’s a great question, even if I’m not able to answer it, so thank you so much for asking!
Makes we wonder where the ubiquitous epithit “flame hair” originated.
Also, i’ve seen Thor be called red beard, but never red hair? In fact, blond people with ginger beards is fairly common, so unless I’ve missed a kenning that’s hair-specific (which tbh is totally probable), Thor’s hair color could go either way.
I found this reference in wikipedia:
“On the red beard and the use of “Redbeard” as an epithet for Thor, see H.R. Ellis Davidson, Gods and Myths of Northern Europe, 1964, repr. Harmondsworth, Middlesex: Penguin, 1990, ISBN 0-14-013627-4, p. 85, citing the Saga of Olaf Tryggvason in Flateyjarbók, Saga of Erik the Red, and Flóamanna saga. The Prologue to the Prose Edda says ambiguously that ‘His hair is more beautiful than gold.’”
It’s also possible that he’s referred to as redbeard not because he actually has a red beard but to emphasize his manliness and virility (white Christ and red Thor, etc.) like calling deities “white” doesn’t refer to skin color.
As for Loki’s “flame hair”, we cannot infer that this is a canonical mythic idea, as there is no proof, Eddas or otherwise, that Loki was worshipped in conjunction with fire (or worshipped at all). This alleged connection is conjecture, as are theories of an association with Svartalfar— which as far as I know are directly related to the theory of his affiliation with fire— and his specific heritage as a Hrímþurs is a theory of “what ifs” as well (while Fárbauti has a typical Jötun name that could be associated with rime giants (striker/crusher), it also lends itself to the sound giant side of things, and Loki’s mother Laufey/Nal has a name demoting earthen imagery, while his brother Helblindi and Byleistr don’t help us much either (what can we derive from bee-lightning)? As far as Thor’s beard, I believe the color is attested only in the sagas. In any event there has been an attempt in scholarship to declare the red color of Thor’s beard separate from eddic influence (as Gef mentioned, it’s discussed by Ellis Davidson and many feel it is an association with the devil). Honestly I think Sif is the only one in the pantheon to have a physical descriptor of a color for her hair.
In any event, I think it’s always important for us to form our own opinions on the gods and their appearances, but it is also extremely important to remember to differentiate between theories formed around the Eddas, sagas, and skaldic verses, and those formed by popular invention of heresy. We can all choose what we want to think (I think Thor has a red beard, why not?) but we should remember that a lot of what we think has no relevant proof within the mythic corpus. For instance many of us think of Loki’s tell-tale horse-form as a white mare, but Snorri does not denote the color of either horse. Horse color dynamics would suggest that a grey goal would be the most likely outcome of a black horse crossed with a white one, but again: inference via modern thought and observation. So ponder and decide, but let’s all be careful about what we state as fact when honestly, most of what we have are theories, some more far-flung than others.
So I hope this isn’t intrusive but since you brought up horse coloring and breeding–
All white horses are gray. The gene causes gradual depigmentation, causing some horses to look pure white by the time they reach full adulthood. Two white horses will usually result in a gray foal! (Unless the foal inherits recessive genes for other colors from both parents.) The gray gene is also dominant, so one white or gray horse and a horse with any other coloration can also result in a gray foal.