Hi Paul, The usual approach for coloring a molecular surface is to color the nearby atoms. The surface is by default colored to match colors of the closest atoms. First you select the atoms near the antigen - antibody interface. I'm not sure what the obvious and simple way is to do that task. But you can select the antigen, then use Tools / Higher Order Structure / Multiscale and press the "Contacts" button. Then color those atoms with the Actions / Color menu. Clear the selection (ctrl click on background) then use Actions / Surface / Show to get the high resolution molecular surfaces. Sounds like you have tried the Surface Color tool. That won't color by distance from a selection (though maybe it should). That operation is done by the Color Zone tool (Tools / Volume Data menu) but that only works on volume contour surfaces, not molecular surfaces. That quirky limitation will be removed in the next release but that is 6 months away. Tom