Just make the min and max brightness symmetrical around zero. For example, in the script below the range for the overlay is -4..4. Note that for most statistical measures values near zero are very noisy. Therefore, it is often a good idea to load a two-tailed statistical map twice, once with a threshold for positive voxels and the second time with a threshold for the negative values. For an example of this approach, see the "Scripting/Templates/Basic" demo. 

import gl
gl.resetdefaults()
#open background image
gl.loadimage('spm152')
#open overlay: show positive regions
gl.overlayload('spmMotor')
gl.minmax(1, -4, 4)
gl.colorname(1,'magma')
gl.opacity(1,50)
gl.orthoviewmm(0,0,0)