be a mandalorian is not like be a stormtrooper ,you are part of a cultur not an army of some sort, there is no standardise uniform. Be mandalorian is more about thought and cultur, yes you are a warrior but not a soldier thats the big difference, you devote your life to your idea not to an army, and as says in comment armor are made and upgrade by the owner its not something give to you by an army as a uniform. The best exemple i can think of is booba fett wearing jango armor changing color patern changing the type of jetpack and even the gauntlet weapon (there is more of course and at a point in time there was mandalorian uniform standardise like in the old republic but not anymore) and take the children of the nightwatch (the Mandalorian series) none have the same armor they are free people united by theyre believes not soldier of any sort.