What you are seeing is known as jaundice (or icterus, medically). The color results from the build-up of certain bile pigments in the blood. Some of these situations can be reversed and some cannot. Your cat needs to have a full physical exam and some blood testing done in order to determine the cause. Then, you would have a better idea if this is reversible.
Jaundice is serious. That's why I suggested a full exam and blood lab work. It's important to find what's causing the jaundice in order to know if it's reversible or not. Jaundice, by itself, won't kill your cat...but, the underlying disease that's causing the jaundice could.
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