The difference between a multicellular organism and a colonial organism is that colonial organism consists of individual one-celled organisms that can, if separated, survive on their own. On the other hand, cells from a multicellular life-form cannot live independently (they are all connected to function together).
Volvox is not considered multicellular because its cells are not aggregated into organized tissues and have no organ and organ systems.
The difference between a multicellular organism and a colonial organism is that individual organisms from a colony can, if separated, survive on their own, while cells from a multicellular lifeform (e.g., cells from a brain) cannot.
Volvox is indeed multicellular because they are a bunch of unicells forming colonies. But they did not give rise to land plants. Another group called the charophytes did! They are green algae, multicellular and possessing some similarities to land plants. For example, they have sporopollenin on their walls.
