an example can be found on AMOSPro_Tutorial:Tutorials/Rainbows.AMOS
SETs up a RAINBOW effect for later display.
Set Rainbow rnumber,colour,length,r$,g$,b$
rnumber is the number of your rainbow from 0 to 3
colour is an index to be assigned to your effect.
length enters the size of the table used to store your rainbow colours from 16 to 65500.
R$,G$,B$ give the red, green and blue intensities of your colour bands in the format (lines,step,count)