I recently discovered an old sheet of snippets obviously copied from a treatise on painting. I am puzzled by the exhortation 'Do not use Ultramarine Blue in mixtures'. Now in Alwyn Crawshaw's Acrylic Painting Course, which I am sampling for a change of medium, Ultramarine Blue is a major constituent of his mixtures. Should I ignore this previous advice?

What a strange thing to have been written. Ultramarine Blue is one of the most popular colours used for mixing by artists, in whatever medium. I can only suggest that you snip that particular statement out of you snippets and use Ultramarine whenever you like!