But the lightsaber Luke got from Obi Wan was lost on Bespin when Vader cut his hand off. Luke made the next lightsaber he used (and it had a green blade, not blue).

“I see you have constructed a new lightsaber. Your skills are complete. Indeed you are powerful, as the Emperor has foreseen.”―Darth Vader examining Luke Skywalker’s lightsaber in Retrun of the Jedi.