It needs to be something at the front of the title, so it's visible on all but the shortest visible part titles.
I'm not keen on recolouring or shading etc, as they can be hard on the eye.
What I'm finding hard to get a good feel for is that all the "ghost copies" are actually equal. There's no "master/slave" relation, so all the parts end up marked. Next, when a make a "real copy", the sequence gets a new (autogenerated) name. Maybe it would be better if that worked different, so that real copies were named "(RC)" then the original sequence name (or "(RCn)" if they needed numbering)? This would also encourage renaming the real copies to something more meaningful. (You can tell I'm an IT Architect sometimes...)
A "Select all parts with this sequence" would help, too, as that would highlight them quickly.

