For an overlay, if you set the "min" and "max" value as the same, then the entire overlay will be drawn with a single color. You can choose Advanced/Scripting and paste the code below into your script and then choose Script/Run. Again, if you have a pre-thresholded image (where some voxels are artificially set to zero), you need to be very careful about whether you decide to smooth your overlays. You can set this using the "Smooth voxel-based images" in the preferences menu. 

BEGIN
RESETDEFAULTS;
AZIMUTHELEVATION(70, 15);
MESHLOAD('BrainMesh_ICBM152Right.mz3');
OVERLAYLOAD('motor_4t95vol.nii.gz');
OVERLAYMINMAX(1,2,2);
OVERLAYLOAD('motor_4t95vol.nii.gz');
OVERLAYMINMAX(2,-1,-1);
COLORBARVISIBLE(true);
OVERLAYTRANSPARENCYONBACKGROUND(25);
END.