I made some edits and tested in Blender (see the link in my signature).
I eliminated overlapping for the building outline and building part, namely:
I deleted the way for the building part
I created a relation for that building part referencing the way used by the building outline
I set the correct total building height (30m) for the building outline (was around 2m)
building:colour is used for building walls, roof:colour is used for a roof
A useful tip how to copy tags only:
select a way or relation with the tags your wish to copy