> If calling the same function with a different argument would be considered 'function coloring', than every function in a program is 'colored' and the word loses its meaning ;)

Well, yes, but in this case the colors (= effects) are actually important. The implications of passing an effect through a system are nontrivial, which is why some languages choose to promote that effect to syntax (Rust) and others choose to make it a latent invariant (Java, with runtime exceptions). Zig chooses another path not unlike Haskell's IO.