The idea is to be educated. Most all the minerals in colored glass come from China.
Trautman went to China long a go to teach them colored glass. This is where his cullet came from. Northstar also now uses some China cullet as they now own Trautman art glass. Greasy glass also uses China cullet
What makes it bad, nothing if it is stable. That is what matters. All China colors are very stable, expect China black. That is never used by a good blower.