Transparent washes vs working opaquely
Above, I’ve painted two side-profiles of the same character to show the difference between using the white of the paper compared to using white out of the tube. The painting on the left was done with transparent washes of black, and the one on the right uses opaque mixtures of black and titanium white.
If you want a nice range of tonal variation, keep some transparency in the work. Use the white of the paper whenever possible: this will give you the brightest, most saturated colours. But if you need to add some accents to a dark area of a piece, use a thicker opaque mixture on a dry brush to avoid paint lifting and muddying them. Here’s a crop of a painting with some fairly bright highlights on a dark background. I needed to go over them several times to achieve good opacity.