All three have different applications.
- SSJ4 goes with the primal Instinct of the Saiyans going alongside the battle without hesitation and following their guts while still retaining the thinking stuff and the rage caused by it. Fusing two of the most iconic aspects of their race, SSJ and Ozaru.
- SSJG is the application of a completely different type of Ki on the user which affects even the body itself becoming more skinny.
- And MUI is the application of the teachings of the Angels, removing the hate and rage and replacing it by calm and focus on the battle and nothing else till the point where thinking at all becomes a problem. This thing goes directly against everything established for the Saiyan race as they are a race focused on sheer rage to power up and their instinct. MUI on the other hand, as I said, is having the body in a complete state of calmness 'till the point where the body is so in the battle that it acts on it's own
So yeah, calling it only hair color change is just ignoring the implications of the forms itself as their concept isn't even close, how you attain it is vastly different in each case and the effects on the form are basically the opposite.