Why can't I see wieght colors unless I use the numpad [ / ] key? Why can't I see weight colors unless I use / key? It seems I can only see the weight colors when the object is isolated but that isn't always practical. How can it work with all items in view?
You can barely see weight paint colors while in the Material shading mode if the shape is not in Local View, you have to switch shading mode to Solid. 

I was going to delete this post but others have asked before why they can't see weight paint colors.