You have a couple of choices. Either paint the walls and ceiling all one color or paint the 3ft walls the wall color (earth toned) and paint the rest a ceiling color. The ceiling color could be 2 shades lighter than the wall.
Painting the sloping walls the ceiling color would give an illusion of height. Keeping the ceiling paint color in the same family as the walls, a couple of shades lighter, would provide some continuity to the room.