Why can't I use a colon when naming / renaming my file on my iMac?

The Apple filesystem HFS naming nomenclature for file paths uses colons as path separators, so entering a colon in a filename in the Finder will immediately change that color to a forward slash. The latter is the UNIX file separator that one encounters on the UNIX command line in the Terminal application.

It is wise to use '_' or '-' for filename punctuation in macOS to be compatible with the Finder and UNIX shell file naming conventions.

Under normal circumstances the colon ( : ) can't be used in Macintosh filenames.

I suggest you use an underscore ( _ ) or hyphen instead.

Check with your professor for other options.