Thanks Glenn! That helps a lot.
The long-press with the brush active does most of what I want (but only with my finger, not my pencil).
I also sort-of worked out how to use the colour picker in the Colour Studio—it's a bit convoluted (having to click and drag the colour picker icon in the Colour Studio in order to 'load' the colour picker colour, and then click the foreground or background colour and then back to the loaded colour picker colour) but at least it gets there in the end.
I still don't understand why the Colour Picker Tool only applies to vector objects though—it seems very counter-intuitive.