Fill Tool - Fill Effect Field
The Fill Effect option on the Fill Tool Infobar defines the way in which the colors change from one to another in a fill. There are three effects available:
Fade
Simply fades from the colors of one object to the next.
Rainbow
This is a more colorful fade following the shortest edge of the HSV color wheel.
For example, a fade from yellow to cyan passes through green.
Alternate Rainbow
This acts in the same way as Rainbow except the colors follow the longest edge of the HSV color wheel.
For example a fade from yellow to cyan passes through red, magenta and blue.