It is possible to get the symbol : COLON U+003A to expand to \colon, instead of the ratio operator, as it is now? The latter is rather rare in pure math, and in addition, it seems to have been reversed in the implementation with ∶ RATIO U+2236, cf. example below.
\NC f\NC\colon A→B \NR % Produces ∶ RATIO U+2236
\NC f\NC: A→B \NR % : COLON U+003A
\NC f\NC ∶ A→B \NR % ∶ RATIO U+2236