My friend asked me a question that I couldn't answer, but I told him I knew where to find out. In traditional mythology Thor had red hair. Is there a reason why in the Marvel Universe that he is a blond? Thank you.
"If the Commander will pardon me, I would think Marvel Thor's costume is less Viking-like than it might have been because Marvel wanted him to look like a superhero. A beard would've made him look older. Possibly he was given yellow hair to contrast with his cape while matching his belt and boot straps, or simply because it was felt to be a more attractive colour."
Point taken, Luke. For that matter, it's entirely possible Stan had both reasons---trademarkability and super-heroey look---in mind.
The reason I doubt it is, in 1962 Lee didn't know how successful his superhero titles were going to be. But to be fair, Thor debuted at around the same time as Ant-Man and Spider-Man, and just ahead of the beginning of the Torch's solo series in Strange Tales. I think that suggests Lee took a decision to give Marvel a superhero line. So he may have been hoping that the line would have similar success to DC's superhero line.
Also, a few years earlier Marvel/Atlas and Charlton had simultaneously published Wyatt Earp titles. The Marvel/Atlas series narrowly started first.
Fox did a Thor series in the Golden Age, about a mortal to whom the god Thor grants his name and powers. His debut story can be seen here. In this story the god has red hair and a moustache, but his superhero protege is clean-shaven, and apparently has long, blond hair.(1)
In this story, also, the superhero Thor and the god Thor wear helmets with side pieces a bit like Marvel Thor's helmet wings. The Thor in Kirby's Tales of the Unexpected story has a horned helmet.
It's tempting to argue that the Fox character influence the look of Marvel's, but the Fox version didn't last long, and was not cover-featured. I can see Lee remembering the series despite this, but will he have imitated the appearance of a version that wasn't successful?
Aside from his Tales of the Unexpected story, Kirby had previously done the mythologically-themed Mercury/Hurricane feature with Simon for Marvel/Timely. Mercury/Hurricane was clean-shaven and had short, blond hair, and a headband with wings. (His haircut is supposed to look Classical.) Fox's Samson, who was much more prominent and longer-lasting than its Thor, was clean-shaven and had long, blond hair.
I've seen it contended that the transformation motif in Marvel's Thor series derives from Fawcett's Captain Marvel: but the Fox Thor apparently transformed too. (It's a little hard to tell in his debut story, but he has long hair as Thor, and short hair in his other identity.)
Incidentally, the Silver Age Marvel Comics Cover Index site seems to have gone the way of all pixels, but Mike of DC Indexes now has a Marvel site with a time machine feature.
(1) He clearly has long, blond hair on the splash page, and he's a blond in his other identity. His hair looks reddish on page 11. In the image at Toonopedia, apparently from a later story, his hair is brown.