You can adjust this in the graphical user interface with Overlays/TransparencyOnBackground

you can also control this in a script with the overlaytransparencyonbackground() function:

begin
resetdefaults;
loadimage('mni152_2009bet');
overlayload('motor');
overlayminmax(1, 2.6, 4);
clipazimuthelevation(0.5, 180, 0);
shadername('edge_phong');
overlaytransparencyonbackground(0);
azimuthelevation(130,15);
end.