Thank you for the reply.
One of the most common causes of an acute color change in the iris is what is referred to as an anterior uveitis. This is a condition in which the blood supply to the eye becomes inflammed for various reasons. It can be due to primary ocular disease or it can be due to a primary systemic disease that is having ocular effects. Often, if it is not an easily identifiable systemic disease identified, the cause is not found. It can progress to further ocular changes and can become painful and even cause vision loss if left untreated, so it shoud definitely be evaluated by a vet.
Of course it is possible that there is something else going on. Cats can develop pigment changes in the iris that can cause a color change, but that is usually a more gradual change. Also hemorrhage in the eyeball itself can occur which can lead to color change and there are various causes for that, some more serious than others, including glaucoma and retinal detachment.
Since she is an older cat, with other untreated systemic disease, I would treat this as an emergency and seek veterinary intervention today. Eye problems can progress quickly and even if she seems comfortable for now, quick action can mean the difference between retaining vision or loss of sight if she is dealing with one of the more serious ocular conditions.
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