When you use additive colors, it is best to use a Red, Green, Blue as the core color for each overlay - the warm and cool colors maps each have mixtures of these colors, making it harder to distinguish. Assuming you are using the latest version of MRIcroGL, the script below loades three overlays with red, green and blue colors. It then shows them as addititve so red+green=yellow and red+green+blue = white

import gl
import sys
gl.resetdefaults()
gl.loadimage('mni152_2009_256')
gl.overlayload('motor')
gl.overlayminmax(1, 1, 1)
gl.overlayload('motor')
gl.overlayminmax(2, 2, 2)
gl.overlayload('motor')
gl.overlayminmax(3, 4, 4)
#use negative value to specify additive overlays:
gl.overlaytransparencyonoverlay(-1)
gl.mosaic('A 40 50 60 S X R 0')