Follow Up - Mud Paint
I reckon that either more and darker pigment will help, or a much much lighter clay slip, or a clay that is the color I want the wall.
I also need to decide whether to proceed using the damp sponge method or the burnishing method.
With the cold weather and duck management being a bit out of the ordinary right now, the next coat of paint will wait. That will give me some time to make my decision.