Hi
Is it possible to do a parametric overlay similar to the option available with Mango?
Andrew

Yes, with Mango a parametric overlay is one which shows both positive and negative peaks:
  http://ric.uthscsa.edu/mango/usingmango_add.html
With both Surf Ice and MRIcroGL you do this by chooising the Overlays/Add function twice, and selecting the same file each time. You will now have two Entries n your Overlays panel (as shown in the screenshot below). Set one of these to have negative min..max range values and the other to have postive ones. You can set your colors as you wish by using the Color pulldown menus, but it is popular to use the Red-Yellow for positive and the Blue-Green gradient for negative values (as shown in the screenshot). One way to try this out is to run the "fmri_painted_surface" script (choose Advanced/Scripting and then choose File/fmri_paint_surface from the Script forms menu):
  https://www.nitrc.org/plugins/mwiki/index.php/surfice:MainPage#Scripting_a_painted_overlay

If you want to view your surface as a mesh (rather than a painted surface), you can use the Advanced/ConvertVoxelwiseDataToMesh command to convert your images. Again, you will do this twice: once with a negative threshold and once with a positive threshold. You can load the two resulting meshes with two calls to the Overlay/Add command. This is shown in the second screenshot below. Again, you can choose the color (here Red and Blue) but note that when you load a mesh you can not set the minimum and maximum range (it is an isosurface, your threshold was applied when you converted the image to a mesh).