I'm trying to do an additive overlay with 3 maps/colors. I'm having a lot of trouble viewing the overlap for all three maps. Is this possible?

For example, I've tried using 3warm and 4cool, which have a nice, clear light pink overlay. When I add a third overlay with blue, it doesn't seem to show the overlap at all on the 3 warm, and there's no difference (at least not that I can see) between the 3warm/4cool overlay and the 3-way overlay. I thought maybe this has to do with it being in a different color group from the other 2, so I tried 1hot (instead of blue), which seems to work a little better but still doesn't really overlay on the 3warm (2 cold is worse).

I know that if, for example, I used red, blue, and green and set the scale to wash out any gradient, the 2-way and 3-way overlays are really clear. But then this is potentially problematic because I lose the gradient, which is informative.

I'd be interested to hear if anyone has encountered this issue or found a good solution for overlaying 3 maps.

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')

Great! Thank you!