Is this model an imported model?
Try changing the reference set of the model, in the assembly navigator right click an object and choose replace reference set.
I have done that and switched each part to Model, entire part, etc.
I is not imported the parts were all individual parts than were added into an assembly.
I tried with some other simpler parts I have created and it seems as soon as I add two parts to an assembly - all these colors start to appear and face are missing
Just thought that you should know that over on Eng-Tips the OP reported that he was using a laptop with low end graphics.
Maybe try tuning your graphics card:-
Start NX and open a new part file.
Preferences->Visualization Performance Preferences->General Graphics:
press Evaluate Graphics Performance
let it run, when finished close NX
Restart NX and try your part again.