It depends on how the coloring is done. If the background is white, the words are black and the :: are gray, the :: won't stand out.

True, but why bother with the symbols in the first place? User experience is just as important for developers reading/writing text and languages are typically created behind closed doors with a small engineering team only.

Guido got a lot right with Python because it’s clean and easy to jump in. Minimal cognitive load. But still very powerful.