There should be a clear suggestion to WGT that when a sleeve of balls is used up during a game the ball can be replaced by a ball of the same TYPE - not necessarily the same type and COLOR. The color should not matter!!! This is irrational.
It is, however, reasonable to require replacement with the same TYPE of ball. This is to prevent strategic manipulation, for example starting a round with a worn-out ball that is helpful for playing one of the early holes (e.g. spin on BPB 2) with a different type of ball with characteristics that are useful for playing subsequent holes (e.g. distance on BPG 7 or 15, just to use a crazy example).
As it is, you are stuck forever with the same color of golf balls of a certain type, or else you risk having to revert to the basic ball in the middle of a round. (OK, you could decide to use up the old color on practice rounds, if it really gets on your nerves....)