My exceedingly excellent tutor said to use a single colour only plus white to help learn about tonal values. She is a really good teacher.
Herewith the results of my efforts: top one is in oils and the bottom one is acrylic both using burnt sienna plus titanium white. (the EU banned flake white)
…all done with a pack of marker pens from W.H.Smith
Many thanks to Cherry Pie for providing the inspiration.