Stamina = yellow also feels like something I would accept without a second thought. Blue, purple, or magenta stamina bars would, by contrast, feel very off-model and take some getting used to.
if there's no mana, blue as whatever "second bar" including stamina would be fiiiine with me maybe?
Like any UI element, I would expect the colour of the stamina and health bars (as well as the magic bar, if applicable) to communicate something about the game. Their specific colour and shape would depend entirely on circumstance, lore and relative importance.
Defaults like "stamina is green", "stamina is light blue"; "health is red", "health is blue, when low, red" can offer hints, but I don't think it's reasonable or useful to expect the colours to be the same across all games.