Board Thread:General Discussion/@comment-27745396-20160629084010/@comment-775152-20160630234958

The way night and day vision work in our world is that there are two different types of receptors in our eyes, called "cones" and "rods." Rods are more receptive to light, while cones are primarily in charge of color. (They both do both, but it's the concentration and type of photorecepters in them that make them different.) When light levels are high, some rods will "shut off" so that the cones can process more color and give better detailed information to the brain. When light levels are low, cones will "shut off" so that rods can do more work, creating less detail but allowing better vision in the dark. This is why it takes a second or two for your eyes to adjust to sudden changes in light levels; the cones and rods are waking up or shutting down as needed.

Animals tend to have fewer cones when compared to humans, and thus have less color vision, and often have more rods overall. In certain animals, most notably cats, a surface at the back of the eye called a "tapetum" reflects light back towards the front of the eye to create constructive interference and amplify additional incoming light.

Isn't nature great?

Short version: If biology in Remnant is similar to our world, Faunus can have perfectly good color vision during the day, but be able to see well at night due to a higher number of cones and the (possible) presence of a tapetum. However, during the night, both their color vision and ability to see detail would be diminished.