FME seems to be having a problem writing the color appearance to solid geometries.I have created a problem report for our development team, and will notify you as soon as this is fixed.
In the meantime, please use a GeometryCoercer after the Extruder to convert the geometry to fme_composite_surface.This will produce a CityGML file with the desired appearance.
Yeah this seems pretty undocumented.I've been trying to get this to work, however, nothing seems to do the trick.Perhaps@DaveAtSafehas some ideas?Could it be a bug - seems like this should be automatic like it is with textures.