In the "Layers" panel, click on the "Options" pull down menu and toggle "Additive Overlay Blending". With additive colors, red plus green will look distinctly yellow
   https://en.wikipedia.org/wiki/Additive_color
For Python scripts, you can use the overlayadditiveblending() function. For example, choose the Scripting/New function and paste the code below:



import gl
gl.resetdefaults()
gl.loadimage('spm152')
gl.overlayadditiveblending(1)
gl.overlayload('spmMotor')
gl.minmax(1, 4, 4)
gl.colorname (2,"1red")
gl.overlayload('spmMotor')
gl.minmax(2, 2, 2)
gl.colorname (2,"2green")
gl.orthoviewmm(37,-14,47)