Tonal value is the effect of how colors change by adding either white or black.
Colors brighten when we add light or white, and darken when we remove the white or add black. The differences in tonal value create contrast. Contrast defines shapes and helps us to see objects more clearly either in nature or in art explorations. Next, you need to decide whether you want the lightest piece to be your background, or whether you want the darkest shade to be your backdrop. Put the piece you decide on aside, so it won't get soiled.