There are two problems with this: 1. Languages with "coloured functions" usually don't inform you of effects (except for Haskell and languages like it). They a

There are two problems with this: 1. Languages with "coloured functions" usually don't inform you of effects (except for Haskell and languages like it). They allow expressing equivalent semantics with the other colour, which doesn't distinguish effects. 2. Expressing anything in the type system always has a cost. That it can express something doesn't mean that it should. The only way to know whether expressing this difference -- assuming you could (which, again, most languages with coloured functions don't do) -- is to empirically show the benefits outweigh the cost, not just hypothesise that they do, which just amounts to "I like it this way." "I like it" is perfectly fine, but it's not the same as "it's a good thing."