This may not be necessary but I thought this could add a little touch of detail. Some animals (including some people) are born with blue eyes that later become darker as time goes on. This is due to lack of melanin that the animal is born with but will eventually gain. Domestic cat kittens, for example, are always born with blue eyes that start getting hints of green/orange/whatever adult eye color the closer they are to adulthood (this usually begins at 6-7 weeks). By the time they are adults t