You can do it either way. It is all what you like personally. I have had the slanted ceilings at one time. In one room, I did the ceiling, and walls alike, and one room, I did the white ceiling, and regular color on the walls. I liked it either way.

If wall/ceiling intersections have good craftsmanship, 2 colors, otherwise do one for all.

The condition of the walls observed in the corner of one of the photos suggests one color in a flat finish. I would address the lighting issue first, it appears there is little or natural light. Talk to a lighting specialist who can recommend the types needed based on room function - bedroom, office - where you need different types and levels for various tasks.