This is a cross-posting of the original concept design for the main character of Moonlight Angel, as rendered by Kyrio. I'm cross-posting it here because... How could I not?

Anyway, the color of the demon's eyes reflects his emotional state, as seen here. This was a trait that was inspired by the dragons and firelizards in the Anne McCaffrey Pern novels. Although the eye colors aren't identical to those ones, the same general principle applies.

There are a lot of them, but the most common ones are...

Blue-to-indigo: Neutral/content
Cyan/Turquoise: Happiness or relaxation
Fuchsia: Discontented/frustrated
Purple: Deep sadness
Red: Rage or bloodlust
Yellow: Fear
Green: We'll see~.

Be sure to follow  kyrio on his own page too. As you can see, in addition to being my partner-in-crime on this undertaking, he's an unbelievably talented artist and comic writer as well. He deserves all of your love!

Be sure to favorite his upload of it as well, here: https://www.furaffinity.net/view/26313849/