Hi zukko,
As I understand it, a surface is a construction element, this is why it's not visible by default in an assembly. It is used to shape or build a solid, and nothing else.
But a surface can be made visible in an assembly. You just have to right-click on the part which has the surface in the PathFinder, and in the contextual menu, select the Show/Hide component sub-menu, and finally select Surfaces.
The surface will show green, but you may be able to change its color with the Face style tool, not sure about that.
(Note that I use a non-english Solid Edge version, the tools names I used may not be exact)