Please note – this is my biology teacher side talking in reference to humans.

Eye color can definitely change during the lifetime of an organism. Hormones released during growth can cause genes (including those that code for eye color pigments) to get turned on or off. Stress can also have the same affect.  This can also happen in only some cells, which is how you can end up with rabbits with spots (genes for pigments are turned on in some cells but not others).  While I have not heard of pigmentation changing in only part of an eye during the lifetime of an organism, it certainly seems with in the realm of possibility.  So my vote would be probably normal and cool to watch, but ask a vet if it makes you nervous or it seems like their vision is worsening.