Welcome to JustAnswer! I am a licensed veterinarian and would be glad to help!
Is there a distinct mass or growth there or just a diffuse color change on an otherwise normally looking iris?
Based on your description, the most likely cause for this is a benign melanosis.An irritation of the iris, an iritis is also possible but a tumor is not likely (iris melanoma)
If you wanted to be 100% certain, this link will direct you to a resource to locate an eye specialist in your area - CLICK HEREI hope this is helpful!_____________________________________________________________________Please click "ACCEPT" if the information I have provided has been of help so I receive credit for my work. Bonuses are always welcome and appreciated. Thank you.The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would highly advise contacting your regular veterinarian.