The available tail styles are ariel, fish, plain, rainbow, and shark:
The style is chosen by the player’s skin having the pixel at (1, 0) set to one of the following colors:
The tail is tinted to the color of the pixel at (0, 0) unless the style is unknown and then the tint is the average color of the body’s front’s bottom eight pixels. This allows for a good tail without the need to change your skin.
An example Ariel skin: