I just found the 3d tagging scheme for JOSM and it helps a lot. Another topic was the 3d tagging of the area of the new vienna main station.
The tags I do add for 3d, if possible: * building=* * building:levels= * building:levels:underground= * building:min_levels= * building:colour= * building:material= * height= (in meter..) * roof:colour= * roof:shape= * roof:orientation= * roof:height= * roof:material= * roof:levels=
For colours I do use this chart: http://www.w3schools.com/html/html_colors.asp
Important is the relation of type=building for all building parts.
EDIT All is noted in the http://wiki.openstreetmap.org/wiki/DE:Simple_3D_Buildings guidelines, supported by the 3 bigger 3D renderer of OSM data.