Here is the actual language from the rule book:
c. One Ball Condition
If it is desired to prohibit changing brands and models of golf balls during a stipulated round, the following condition is recommended:
During a stipulated round, the balls a player plays must be of the same brand and model as detailed by a single entry on the current List of Conforming Golf Balls.
Note: If a ball of a different brand and/or model is dropped or placed it may be lifted, without penalty, and the player must then proceed by dropping or placing a proper ball (Rule 20-6).
As far as I can recollect I would have only said a PGA player can do that if the special rule was not adopted. It is generally adopted only in events that are limited to professional golfers or highly-skilled amateur golfers. Generally, this condition of competition is not adopted in club-level competitions.
If you really want to get into it here is the current list of conforming ball types. In the how to use the list you will note that each ball type is listed as a separate entity. Among those separate entities are balls that are in a different colour (note I spell color correctly though...could not resist sorry). Thus, any which way, the colour of a ball being different makes it a separate single entity on the list (list uses the term entry). Per the rule above we have it that a ball must be of the same single entry (e.g. a different colour is a different ball type for our purposes).
I know you said letting go, but did this anyway so please forgive.
I also agree it would not make bat crap difference here although if I were WGT I would not change it. I can only imagine the uproar the first time some one lost real credits because of a vapour trail or whatever the argument would be. Human nature those last parts:). Anyway for now is what is, and until one day the stuffy USGA / R & A governing bodies rules get relaxed / changed is what is.
Geeky side of me found this interesting anyway.
ShadyShank: 1.92 points to drop to get my L thanks to yours and YankeeJims advice. T
For my small part you are most welcome. I was glad when people helped me with what to work with (Lizard69 hugely). Only time now, and will be well deserved:)