Hey Sophie, I’ve been enjoying your thread and sketches, keep it up!
Fairly early on, pjbowers hit me with a great suggestion of sketching with a blue indigo prismacolor premier pencil. Before then, I had only been sketching with a black felt tip pen and the different values you could get with the pencil really taught me a lot about shading, shadows and highlights.
I’d also recommend this as a fun exercise to try because you’re limited to one color and one tool to create all the values and line weight. I think there’s something to using blue pencil because I think the lack of huge contrast that you’d get from a black pencil forced me to look more critically at the form, perspective, and detail of the object I was sketching instead of tricking my eyes to be satisfied by using other colors and contrast to make the sketch pop. You’ll always have those tools for a sketch you’d show to others so this is more for practice!