The official information states that, provided you do not stack the Wonderful Pieces that you obtain, Charlie may make a Wonderful for you that is NOT Purple. If Wonderful Pieces are stacked, however, the default colour always will be Purple.
Consequently, I meticulously kept every Wonderful Piece separate that I obtained either from a Festival or as a purchase from Chen's Market. No matter what I did, however, I always obtained a Purple Wonderful.
I now know why this occurred. The fact of the matter is that, ALL Wonderful Pieces obtained as prizes at Festivals are Purple irrespective of the colour of Wonderful that you would have obtained had you won 1st Prize. The only Wonderful Pieces that have a chance of being another colour are those purchased at Chen's Market. You therefore can stack all the Wonderful Pieces won at Festivals, but make certain that you keep any Wonderfuls purchased from Chen's Market separate (and remember which slots they occupy as well if you ever wish to be able to obtain a different colour from Charlie.)
The fact that only Wonderful Pieces purchased from Chen had the chance of creating other colours is an important piece of information that was lacking in my Guides until now.