I'm watching THE SAVAGE (1952) this morning on Amazon Prime. It was Charlton Heston's second Hollywood film at Paramount, and he's very young. He plays a white man whose family was killed by the Crow Indians when he was a boy. The Sioux came along and took him into their tribe, so he grew up as an adopted member of the Sioux.
The filmmakers have smeared some sort of brown oil on Heston's face (unless it's just his deeply tanned skin), and he fits in with the Sioux. But he's supposed to be adopted, so even if he didn't fit in entirely, it would make sense.
On the other hand, most of the Sioux in this movie are played by Caucasian actors. And they do not seem to fit into this picture. Don Porter, who costarred as Ann Sothern's boss on her sitcoms, and later turned up as Gidget's father on TV, is one of these performers. He is not very convincing at all as a full-blooded native.
I'm not sure I understand the reason Hollywood studios did not cast native actors in major native roles. This particular production, THE SAVAGE, was filmed on location in South Dakota. And supposedly, the producers did use real native extras. Plus Sitting Bull's 91 year old son has a small part in the story. But when it comes to the main native roles, they are done by whites.
Another reason was the studio-system and the fact actors were under contract and where paid regardless of how many films they were in. Stand-ins or background actors were paid a very low rate and where hired per picture, but even secondary actors were mostly contract players.
Therefore a director was told to use a warm body under contract instead of hiring them. The Make-up department did the rest.