I would color Princess Luna in a gradient of blues. I would start off with a light blue for her coat and darken the colors as I move up her mane and tail. To create depth, I would add darker colors to the backs of her legs, wings, and tail. To really bring life to the image, I would use a combination of shadows and highlights to define her facial features and hooves. I would also add highlights to her mane and tail to make them appear wispier and more magical. Finally, I would add a soft white to her crescent moon cutie mark to give it a subtle glow. All of these colors together will make Princess Luna look serene and majestic!