as I've been painting more very dark, almost black areas (sign of the times?), I've used ultramarine combined with burnt umber and tried to keep the layers fat over lean. I'm also careful to avoid white/light paint in the brush. Perhaps there´s been been a few days between layers, and what happens is that the pigment in the darkest areas sink into the previous layer and becomes very matte (lighter and less saturated). I've then oiled it out with an instant result, but when this oil dries a couple of days later, it quite often becomes matte all over again. I've noticed replacing umber with another color seemed to help, so perhaps it's has something to do with the umber.
Have you experienced this phenomena, and how do you avoid it?
For me it's been quite a struggle at times...