Jo, do you want to do a large mural? If so, I would re-think using watercolors since any splashes of water from the shower/sink, etc. can ruin your mural unless you use some kind of fixative or varnish or other finish after it's done. This is because your "reactivate" watercolor paint by adding more water to dilute it and then it can run. But if you do, I suggest starting small and see how it holds up. Or consider using acrylic paints that won't run when there is moisture.
here are some ideas it will need sealed because of moisture in the room really nice https://www.youtube.com/watch?v=CYMXVmep_TY more ideas https://www.youtube.com/results?search_query=diy+watercolormurals even more https://www.youtube.com/results?search_query=diy+watercolor+wall+ another https://www.youtube.com/watch?v=ggyh-wONG40 another https://www.youtube.com/watch?v=CYMXVmep_TY
Hi Jo! Take a look at these ideas! Have fun.
You can paint plain walls with water color but you would have to seal it with paint sealer or resin or something. I don't know how the resin would work, but there is spray for sealants in paint stores