A better solution may be to _not_ use the : syntax at all.
I've recently crafted an API for non-programmers, and decided to make
the whole object system using dot notation. Exactly for the reason of
not confusing the users on when they need dot, when colon. It's always
dot.
This is performed by sending the 'this' object as upvalue to all the
methods. It may give a slight runtime penalty but I doubt it's even
measurable. Can show you sample if you are interested, but the
technique has been mentioned on the list before.
-asko