Whatever your painting style, there are sage comments about establishing strong lights and darks:
“In a figure, look for the highlights and the deepest shadows: the rest will come naturally, often it is very little.” Édouard Manet
“Remember the enemy of all painting is gray: a painting will almost always appear grayer than it is, on account of its oblique position under the light.” Eugene Delacroix
“Trust your eye: if you see light, paint it. If a composition is predominately in shade, perhaps that’s where your eye goes and burning out light areas is warranted.” Mike Carroll
Finally, and succinctly stated by Bob Ross: “You need the dark in order to show the light.”
Break-a-Brush!
David