One color. Since you say you are doing this and reselling the buyers don't want anything too busy. They want 'simple' so they can put their personal touches into the room when they move in. Accent walls, sponge painting, borders, ect., are no-gos when it comes to putting a home on the market.

Oil primer is best for a bathroom where there is moisture. I've used Rustoleum spray primer in that exact application. But whether you spray or brush, oil is best. Then paint.

What are other colors/patterns in the home? I am an avid fan of developing a cohesive and unified look throughout the home. That's not to say matchy matchy, but certainly be aware of the other colors/patterns in the home and find a dedicated connection. Paint stores have brochures that show how this can be done - on the internet also, of course. At the paint store you can also ask questions for a solution for your dilemma. I have a very eclectic look in my home, but it centers around modern art prints. Although there are a lot of different colors in my home, the consistency is that the walls are always off-white and the art is always modern.

I'll be a contrarian. I would never knowingly rent a room with a shared bathroom in the hall. That would be a complete pass - I don't even like having to use a hallway bathroom in a sleeping situation in which I would share with friends and family because if I need to use it in the night I would have to wake up more/get more decently dressed and I hate having to shlep stuff to take a shower or perform normal grooming stuff in a bathroom. I don't have the horror of viewing a toilet from a bedroom that some houzzers seem to have. It certainly wouldn't bother me on a temporary basis. I have a master bath in my home and I can't remember the last time I actually shut the door. I don't have issues with smells or humidity but perhaps that is because my Toto washlet has a built in deodorizer AND I have an excellent Panasonic fan which runs based on ambient humidity levels. I don't generally even have condensation on shower doors or mirrors after exiting let along huge amounts of humidity wafting through my bedroom. The question really is one of economics. There is a cost to installing the bathroom and OP would need to decide whether they would make up the expense by having increased rent and/or renting it out more frequently. Certainly any prospective renter would see a picture and determine whether they wanted the ensuite with a curtain in the room or use of a bathroom in the hall. Also resale value is a red herring. At such point that OP wanted to sell it would be relatively inexpensive to convert the bathroom into a closet if having the ensuite in that room was a negative.