Coloured functions are a feature - not a bug, Haskell is full of them, and they are exactly what make STM safe in Haskell, but abandonware in other languages which have tried.
2. The way you call a function depends on its color.
3. You can only call a red function from within another red function.
IO can call IO functions.
IO can call STM functions. (*)
IO can call pure functions.
STM can call STM functions.
STM can call pure functions.
pure functions can call pure functions.
Having these coloured functions is what made STM achievable back in the mid-late 2000s, since the mechanism to prevent STM or pure functions from calling IO was already in-place.
Other languages either tried to figure out how to contain the side-effects and gave up, or just released STM and put the onus on the user not to use side effects.
It is a shame that the people you are answering is being downvoted, I also understand the importance of coloring functions, but look at the examples that person put, python and rust. In those, executing a colored function (at least the async related ones) propagates up to the top of the program, that is a cost that we have to interiorize, but I would be lying if I told you I wouldn't he happy with such behavior. I do a lot of js/ts and I would love to just be able to "inline" await without making my current scope recursively to the top of the program like it can be done with F# with the Async.StartAsTask operation.