an example can be found on AMOSPro_Tutorial:Tutorials/Rainbows.AMOS
Creates an attractive RAINBOW effect using the values you've defined using SET RAINBOW or the RAIN function.
rnumber is the number of the rainbow from 0 to 3.
base is an offset to the first colour in the rainbow table to be displayed on the screen.
y enters the vertical position of the rainbow as a hardware coordinate.
height sets the height of the rainbow in scan lines.