Learn more from this SciShow video about Norwegian reindeer with eyes that can change color with the seasons. Researchers found that this unique skill has a special purpose; in the low light of the Arctic region, the retinas of the reindeer become slightly more sensitive.
Some animals have a layer of tissue behind their retinas called the tapetum lucid. It is what causes animal's eyes to seem to glow at night when light shines on them; the layer is reflecting the light back. While most animals eyes appear yellow when light shines on them, the reindeer's eyes turn blue in the winter. Researchers found that it seems to enable the animals to see a bit more, although it may make what they see a little unfocused.