Parts are 'Blacked Out' when being assembled

Hello,

I am hoping someone can help me with a problem I have been having for a while now. So far it has not been a huge problem to deal with but recently, with more intuitive models, I am starting to struggle putting things together.

When I first started using CREO this wasn't an issue and I dont know whether it was something I have done but now every time I want to assemble a part in an assembly or an assembly into another, the whole model turns black.

I have attached a picture for reference. The grey model is the assembly and the black part is what I want to assemble onto it.