How to undo  in Object Mode? Green color selection I'm an absolute beginner in Blender. Now I've accidentally pressed Ctrl+G in Object Mode, the selection color turned into Green selection. How could I make it back to Yellow / Orange color please?

I've been through this Blender Hotkey Wiki:
https://en.wikibooks.org/wiki/Blender_3D:_HotKeys/3D_View/Object_Mode and it says here: 


  Ctrl+G brings up menu for performing group operations on selected objects.


But I din't see any menu. Any helpful answer would be appreciated. Thanks.
Ctrl+G adds selected objects to a new Group

Objects belonging to a group show up in green.

Shift+Alt+G removes active objects from a group.

If more than one object is selected and when you press Shift+Alt+G but only one of them is removed from all groups, then afterwards press Ctrl+L > Groups to remove all selected objects from all groups.