The wood is rough sawn . Paint an off white let it dry then take a stain and take a rag and wipe the walls with a kinda dry rag you know wipe a good size area so it's just colored. You could let that dry then with the same stain dip a tooth brush and take like a Popsicle stick and flick spots all over it as thick as you like then varnish the walls . There are lots of things you can try if you don't like it you can always paint it again with the stain you could use a paint brush just dip the brush and dap it on a rag then lightly brush the walls in one direction streaking it
You can use the airbrush technique whereby you will be required use the freehand exploitation of the airbrush. You will spray the walls in order to create a definite predictable consequence on a constant foundation with or exclusive of shields or stencils.