I came across a striking picture of handpainted wallpaper with white flowering branches on a brown background (here, the 11th picture down). It occurred to me that a similar effect could be made by drawing on painted walls with white chalk, and that this would be a reversible and renter-friendly option for dealing with some of my mud-toned walls.
So I've been experimenting with several colors of chalkboard chalk. Chalk is, of course, not as opaque as paint, so there is much less contrast. Nor is chalk anywhere near as permanent; I fully expect some children to come through in the next five minutes and wipe out a big chunk of my efforts. I was able to almost completely erase the chalk with just my sleeve; I expect the rest will come off with a little scrubbing with a damp rag.
The lighting in here is terrible, and my drawing skills are not much better, but here it is: