Vader was able to turn on Luke's green lightsaber in Return of the Jedi. He didn't try to activate it and have big letters show up saying YOU ARE NOT ABLE TO EQUIP THIS WEAPON.
And before Anakin fell to the dark side he used Dooku's own lightsaber (which was red) to cut his head off. (I saved this one for second bc I know the argument that he was beginning to fall would be made.