Welcome to the forum, Stephan.
You might be having a problem with your selection filter. You can easily select individual surfaces for appearance changes at part level and assembly levels. The only limitation is that if a surface is broken up but made as one feature, it will remain grouped unless you have the advanced surface model extension.
Try setting the selection filter to "geometry" and select the surface you want to change the color of.
There is also some limitations in the animation tool. If you use transparency in the sequence, it does not act on surfaces with appearance changes.