Rudolph the reindeer is best known for his brightly colored red nose. For other reindeer, however, the most colorful thing about them is actually their eyes. Researchers have found that depending on the season, the eyes of Arctic reindeer change from gold to blue.
Tapetum Lucidum
Like many mammals, Arctic reindeer have a structure in the eye called the tapetum lucidum, TL for short. It sits behind the retina and reflects light back to increase the sensitivity of night vision. The TL is also responsible for reindeer eye color changes.
During the Arctic summer, when it's always light outside, reindeer TL is gold a color that reflects more light back through the retina, better enabling reindeer to see in daylight. (This is why, by the way, many mammals have gold colored eyes.)
When Darkness Reigns
But when the Arctic winter sets in and darkness reigns, the TL turns blue. The result of this is less light reflecting back out through the retina, making it easier for the animals to see predators in dim conditions.
The color change may be caused by shifting pressure in reindeer eyes, which stems from constant pupil dilation. When compressed, the TL reflects shorter wavelengths of blue light which are more plentiful during Arctic winters.