Protanopia

The Jotun species had evolved over time to fit the harsh cold of their climate. Long luscious eyelashes, a sensitivity to the sun and a form of colourblindness considering Jotunheim was rather... Dull.
When Loki decided not to tell anyone he was colourblind just to see how long he could keep it a secret, he assumed it would be revealed grandly or perhaps in some other surprising, hilarious way. Not when his life was on the line.