When I've encountered colored functions(and here I'm specifically thinking of D's pure annotation) I have noticed that the architecture tends to collapse to the most permissive forms very quickly, because you will discover layers that were meant to encapsulate the coloration, and did "quack like it", but in a strict sense do not.

For example, one's expectations of a pure function would mean that you could do some arbitrary numeric computation on inputs and return a value. Unfortunately, the compiler is not so naive. If you should do some transcendental math and call a standard library function to do so, you have manipulated hardware state. You are now impure, and so are all your callees.

And so you have to decide at that point: is the code you are writing wrong? Or is it just using too coarse an assumption for its invariants?

A huge amount of what determines our code architectures are the way in which we've organized our own thoughts. In this respect we always go from looser to tighter specification over time as the code matures, until tightness has produced brittleness and cracks. I believe coloration forces some tightening decisions to be scheduled earlier. In that respect it should be treated with some suspicion, as it may produce surprising couplings.