The colour of the underpainting or 'ground' can influence the whole atmosphere of a painting. Usually only tiny snippets of the underpainting show through by the end of a completed painting.
But it is a good exercise to prepare a number of sheets with different underpainted colours and use these for simple tonal exercises using charcoal for the darks and wax pencil for the highlights. The ground colour provides the mid-tones.
Here is that frog again, done on an ochre ground and a burnt sienna ground.
These are just on paper stretched and sealed with a couple of coats of gesso and then a wash of the colour in acrylic. It is easy to smudge the charcoal so I periodically sprayed the work with a fixative, allowing it to dry before continuing to work.