A half bathroom is a space that only contains the bathroom vanity and the toilet. Because it's such a small area, people often end up taking it for granted. Instead of it existing as that closet-sized space under the stairs, why not spruce it up?
Look at the small square footage of the room as an advantage - this means you'll be able to finish your project faster and it won't cost as much to renovate. In a half bath, the bathroom vanity becomes the main focal point, so put in some extra effort when redoing this piece.
When it comes to the color of the walls, try to choose light tones that will make the room seem bigger (this is especially important for half baths without windows). Just make sure that the color that you choose will complement the rest of your bathroom accessories. Additionally, because of the lack of space, it's best that you make good use of the walls when it comes to decorating and storage. Hang shelving units for your knickknacks so they won't crowd the floor.