Requested by @magmasteelix
Unlike the Muks in Kanto, Alolan form Muk is strikingly colorful. Alolan Muk looks like a colorful oil slick. Soap, petrol, grease, and other oily substances often look like colorful rainbows. But why?
Oil is both reflective and refractive of light. When light hits it, some of the light bounces off the surface and directly to your eye. The rest of the light passes through the oil, and then bounces off of whatever surface is beneath the oil (water, the road, Muk’s body) before going to your eye. Passing through the oil bends the light a little bit. It’s a lot easier to walk through air than through water: you move slower when you’re in a swimming pool. Light is the same way, it slows down in water and oil, which causes it to bend.
Because of this effect, you can think of it as one beam of light going to the oil, and two beams of light coming off of it: one from the surface reflection, and the one that passed through the oil.
We don’t see two different beams leaving the oil, of course: the two beams combine together to create a new light beam altogether, through interference. Interference can make light stronger or weaker, depending on how the waves add together.
White light, of course, actually contains all of the colors of the rainbow. When light refracts and reflects off of the oil, different colors are going to combine through interference. Depending on the depth of the oil slick in different places, red might combine with red to create a really strong red, and so on. This constructive interference creates the rainbows in oil.
This is how Alolan Muk is so colorful: the outside of its body is coated in a layer of fatty oil, which interacts with the light in this way to make it colorful.