Solidworks - Materials&Appearances Turning Grey? (Not Default Grey)

Hello there,

A quick queries but i am having issues with appearances.

With assemblies I can apply materials/appearance to parts and bodies etc... Now and then the whole thing will go grey.  The appearances are still there in the tree and there isn't another default material over the top.

Sometimes it will come back if I close the whole assembly and sun assemblies that are related to it and restart.  But most of the time it involves shutting down soliworks and rebooting it....

Has anybody else come across this at all?

SolidworksPhotoview 360