Hello Nahzia,
Welcome to the HP Support Forums! :) I understand when you are printing pdf documents that are in color to your HP OfficeJet Pro 8600 they are printing in black and white only. Can you please let me know the Exact model of 8600 you are using as well as the current Operating system you are running MAC or Windows?
Also what program are you printing the pdfs from? Please reply back with the requested information so we can begin!
Thanks,