At one point in the 1990’s (I think, could have been the 1980’s) designer Terence COnran did a lot of work using non-typical finishes for interiors. He had a penchant for making spaces look as if the plaster had fallen off, or water had stained the walls, but always in an artistically “right” pattern. I doubt that he is still doing this sort of thing, but some of his older books might show it. If not, you might consider looking up a copy of Terence Conran Small Spaces : Inspiring Ideas and Creative Solutions. If you can’t find that, then look for The Essential House Book, Getting Back to Basics to get some ideas.
If there is no drywall up, you could either paint the wall surface with a solid color, to minimize the differences between support beams and flat surfaces, or you could exaggerate and put in a wall of shallow shelving, painted in contrasting colors, or you could plan to put something up to cover the walls, such as inexpensive floor-to-ceiling curtains or paneling.
One thing I like to do is go to a fabric store and look for fabrics I like that have several colors in the design. I then buy some of the fabric and copy those colors and the proportions and apply them to the room and its furnishings.