The vinyl stickers (like for window decals) work GREAT! You can usually get them cheap enough. If you dont want to buy the decals to dye with invest a couple bucks in shipping tape. The wide clear tape. I've found I can easily tape the entire disc, then use a boxcutter (razor-knife) to cut the areas I want dyed. I used tape to mask a
confederate flag dye job that went well. take a look The lines visible are actually the mini-blinds behind the disc as I held it up to the window for picture. If you use the tape masking method be patient removing it. Just peel off a section and ease it off in strips.
The acetone works really well to darken and accelerate the dying process. What would take a few hours using water and rit dye would take minutes using acetone... Fumes reek though!