Seems like the main objective of Level 1-3 is to get the Yellow Can and then revert back to where you came from.
if you referring the stage 3, you cant ''beat'' the level at the moment, in the room with harpy graffiti and two blocks (red and green) is a dead-end, it's intended to halt player from progressing
bring the yellow spray can to stage 2 and beat the secret boss in there (afterward you can visit two extra stages from what i heard)