After a quick search in draft N4296, I could not find any example of a name in the C++ Standard Library for which two possible spellings exists (BrE vs. AmE).
While this may even be intentional, I can imagine that at some point, if a graphics library will be standardized (and there seems to be some effort going in this direction), the choice between "colour" and "color" will have to be made.
Are there normative regulations or perhaps even informal criteria that are being used to decide which spelling to pick for names in the C++ Standard Library?