Luciano,
Interesting. The principle (R&A rule 15-1) is that you are supposed to play the same ball from tee to green of the same hole. You are allowed to substitute a new ball only under certain circumstances between teeing off and holing out, such as when it breaks or is lost etc., but there is no rule concerning similarity between types of balls when you do that.
So normally in real life you are free to change golf balls from one hole to the next.
If the "One Ball" condition or stipulation is used for a competition, then the player must use the same "brand and model" of ball during the round. It would seem to me that the 'brand' would be 'WGT' and the model would 'GI-SD' or what have you, regardless of the color.
However, the USGA annex concerning the One Ball condition reads as follows:
'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:
"Limitation on Balls Used During Round
(Note to Rule 5-1):
(i) One Ball Condition
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."'
The List itself includes a separate entry for each color of the same golf ball, so I guess that settles it.
But does golf really have to be all that technical? Have we lost something here?