Partially Brown eye

This is one of my other cats her name is Boba She's around 5 almost 6 years old and I've had her since she was 5 weeks old From what I can remember she's almost always had a little speck of brown in her left eye Today I noticed it looks a lot darker and more covered then it had been the last time I really looked at her eye. From what I've read up on this could be a number of things but I'd like to attempt to narrow it down The first two pictures are the newest and the rest are older I do work alongside vets so I'll be able to take her in and get a more conclusive idea on Monday when I go back into the clinic but I'm just curious

Hello!  As a vet, it always makes me nervous when someone adds, 'oh yeah, and I'm in the profession....'  even though the truth is, that it shouldnt make any difference at all.... still, I'll get my excuses in early.  I, like most teams, am a GP not a specialist, plus obviously Im looking at a photo, with all the confusions that that can potentially involve.   However.....

Colour changes in the iris often reflect uveal tract diseases and to my understanding one of the more common of these is cancer, although other possibilities may include clotting disorders caused by many different things,  parasites and also infection.   We're very pleased that you're seeing your vet who can view this from different angles and - if they're in any doubt as to what it is - may send you on to an opthalmologist.   Please will you let us know how Boba gets on?

I should also mention uveal cysts (look like cancer)  and the fact that your vet can see this in three dimensions, which we can't from here.  It is time to look into this, not time to worry.  As if anyone can avoid worrying to order - but you're doing the right thing getting it checked out.  Thinking of you and Boba.

I have reread your comment that the changes appear to have got worse very quickly.  As in any example of rapid deterioration but particularly the eye, please mention it to your emergency vet for them to triage.