I know this was started last year, but hopefully I'm not too late. Firstly, I admire your ingenuity, Shark13, but I don't think that any dye will overpower the black of the polymer. Secondly, I do believe that if an exchange (or selling it like shepherd77 suggested) for the FDE stock isn't an option, look into some sort of coating like 27 Beck suggested. Firearm coatings are still new to me, so I couldn't argue which is best for polymer.
Worst case scenario, just keep/use the black stock. I realize that's not the original plan, but it seems to be the most cost-effective. Please let us know which route you do take, Shark13.