First, heres a rundown on the biology behind colors. This will help those who have plans to breed bettas and it also helps explain how colors come about. Like humans, fish color depends on the density of pigment in the skin. People with light skin have less pigment and people with dark skin have more. What makes bettas, and other fish, slightly different is that they have 4 different layers that control different colors. They have to have this because fish have so many different colors (humans are just a varying amount of the same color). The most important aspect in fish is that the top layers are dominant to the bottom layers; if the top most layer is a solid color then you wont see the layers underneath but those layers might still be carrying color.
1. Iridescent layer (top layer):
This layer is known as the blue layer and controls the amount of blue pigment. If there is pigment in this layer then you can have green, blue and opaque white