My cat, Leon, has FeLV. He is currently on antibiotic tablets to treat a skin infection behind his left ear. I have recently noticed that his right eye appears a darker orange than the other. When I looked closer, there are small thin red lines on the colored part of his eye, and a few weeks ago there was a dark gray goo coming from both eyes, but the discharge is gone now. Should I be concerned?
It's impossible for me to know without seeing your kitty in person or at least seeing a picture of your cat's eye, but this does sound like cause of concern. Ordinarily, thin red lines are present when a cat has an eye ulcer or uveitis, which is inflammation of the front part of the eye. The red lines are actually blood vessels which are becoming more visible because increased blood flow is an attempt to heal the ulcer or inflammation.
I would recommend bringing your cat to a veterinarian for a thorough ophthalmological exam. It is possible that your cat may need further treatment beyond the oral antibiotics he is already on for his skin infection.
All the best,