Apologies, I'm fairly new to Archicad. I have imported a colour version of an Ordnance Survey map to Archicad 21, and it looks perfect, exactly as it should. When I explode it however, everything goes black, and although I can select objects within the exploded file, I can't find a way to edit the fills. It's all the colour fills that are turning to black, but there doesn't seem to be any way to change them.
The original DWG was probably set to the "existing" renovation filter and therefore all of the exploded elements will be existing by default as well and will probably appear black according to the renovation filter settings. Try changing everything to new or changing the renovation filter.