Hello richardwez,
Welcome to the HP Forums!
I understand your Officejet 8500 is not printing Cyan ink. The only thing I can suggest is following this entire HP document on Fixing Ink Streaks, Faded Prints, and Other Common Print Quality Problems.
Which ink cartridge are running low? How long have you had these ink cartridges installed for? Have you tried changing the ink cartridges that are low, to verify if you're able to print Cyan ink?
Please post your results, as I will be looking forward to hearing from you. :smileyhappy: