One paint color and we have no clue. Paint color is derived from soft furnishings......a rug, a couch. Or art. It is empty of all.

The underside of stairs is just one more wall.

Paint it white until you know more of the furnishings you want or show what you are KEEPING. Which white ? No idea. You test a few: )

Put together some idea books of rooms that appeal, or rooms that have similar styles of furniture that you have to develop a sense of what you like. Then when you have some ideas, paint some large scale test pages ie as big as you can, and paint them . Pop them up and move them round over a couple of weeks and see how the light works with them over that time. The current scheme is liveable for the moment so you can take time to find the better scheme for the room that you feel good in.

We will need mor einfo but I do know how it is to go from everyones cast offs when we were in Uni and start making out home ours. I agree some idea boks where toy both agree on what you post. I would agree to paint it all one color I do not think white walls are great all the time and featire walls are decided after all the other lelements are in the space . Show us what you have that you are keeping in pics here in comments DO NOT start another post . I like this color for an all over wall color that works with almost every other color , I cut it in your 50% so just a hint of color it is BM . I think after loving saturated colors you will enjoy the grightness of this.