I don’t know if anyone has ever thought of this before (besides Tolkein), but you know how Gandalf is grey before he is white? And his white version is more powerful than his grey? Well, think about this:
Our hair is darker when we are younger, and grows white, becoming “salt and pepper” (or “gray”) in between.
And Gandalf’s powers are all mental; not physical.
It’s almost as if he chose those colors as an homage to age and wisdom.
And if you’re wondering why he was never black, that’s because black is a color of evil, and Tolkein didn’t want anyone to mistakenly think that he was somehow evil (that’s just my amateur opinion).
I guess he could’ve been brown, though.
Being red or “blonde” would’ve been too confusing for the reader, I think.
And he had to be grey instead of brown (in my opinion) to stand out from the hobbits.
Again, all just interesting, thought-provoking opinion…