Mac OS 9 and earlier used colons as path separators and some support for that made it into OS X, although it might be gone by now. Apple's guidelines https://support.apple.com/en-us/HT202808 recommend not using colons.
It's not really gone, and is the path separator in the "UI layer" (Cocoa&al.) but the POSIX layer uses `/`. This gets automatically mapped back and forth.
Try to create a file or folder containing `/` in the UI (Textedit, Finder) and look at its name via the terminal. Now, `touch foo:bar` and look at it in the Finder.
