There actually is a tiny bit of reasoning for The Pie being used at stud in Enid Bagnold’s book: p49 of my Scholastic disintegrating paperback:
“…Shilling for a mad piebald gelding. Or is it a stallion, Mr. Croom? Not clean gelded, eh? Thought as much. Mr. Ede done it on the cheap an’ left a chip.” Of course, I doubt the writers of International Velvet even looked into it that much, but I always remember the references to The Pie’s gelding not being clean (it’s mentioned more than once to explain his wildness in the book) when people bring up the issue re: Int’l Velvet.
As Hej said, the color of The Pie in International Velvet doesn’t bother me because the chestnut in the original movie is hardly a color match for The Piebald of the original book. I.e., the color was already wrong, so I can’t get worked up over whether the non-piebald The Pie is chestnut or bay. Just me.
Re: Sylvester, the novelization of the movie (pretty good, written by A.C. Crispin) has the qualifying for the Rolex scenes/competitions, and a big obstacle to qualifying was Sylvester’s balking about water complexes, which was fleshed out more than the few (one?) scenes of him refusing water that were shown in the theatrical release.
None of this is to say either movie is fantastic, but I do love them, and there are far worse, like Lightning or the newer Flicka. Those are so ridiculous as to be unwatchable imo.