The node would render black in this case (but should use the 'active_render' layer choosen in the object data properties -- this is now in line to how this is handled for e.g. UVs) This introduces ATTR_STD_VERTEX_COLOR and uses this thoughout, if no particular layer is specified in the node. Maniphest Tasks: T73938 Differential Revision: https://developer.blender.org/D6887