Do any pieces of PC-12 painted fabric still exist for comparison to PC_10?
Yes, plenty. Most are in the collections of museums and collectors (like Peter Jackson), who do not let ordinary people like us look at them!
But the biggest problem you have is that you do not know what you are looking at. The fabric is not stamped or marked with "this is PC10" or "this is PC12". We can only make educated guesses at what colours were used on each aircraft and therefore if it is PC10 or PC12.
Like I said earlier in this thread, late war Sopwith Snipes were "supposed" to be brown PC12 but Barker's Snipe has a distinctly green look to it, which really surprised me when I saw it. This is an original artifact that was recovered after his air combat and was designated an official War Trophy. It has been kept in the Ottawa museum ever since so is going to be pretty close to the original colour.
IMO it really isn't possible to say for sure what these colours actually were. And there would have been a lot of variation because of the production methods. So don't worry too much and make your own conclusions.