Hi Andrew, We have already implemented capping of clipped surfaces. When you cut the surface with a clip plane it appears solid. The cap can be made a different color, or you can colormap volume data onto it. This is not yet released, but we plan to make a snapshot within 2 to 3 weeks that will include it. It only works for molecular, volume and multiscale surfaces, not ribbons, spheres or other molecular model representations. The phage T4 slab in the Chimera image gallery illustrates the capping. Also the semliki forest virus images shows it. http://www.cgl.ucsf.edu/chimera/ImageGallery/ If you have the above capping is it still important to have different interior and exterior surface colors? We added the capping because views of the interior of a surface don't seem very useful. Tom