can i paint my bathtub wall?
Any Sheetrock, including green moisture resistant, is not intended for use in showers or any environment with repeated direct water contact. You can paint it , but the results will be the same, FAILURE !  Do not attempt to put tile on Drywall either. There are some new high tech backings or you can use good old fashion concrete board or hardi-backer for tiles.  If you want a water proof glossy finish, you may have to consider a fiberglass surround kit, which can be installed over green sheetrock or preferably directly over studs with a vapor barrier.