Just playing around with acrylic over charcoal. I first 'fixed' the charcoal to the paper with cheap hair spray, then applied the acrylic paint. The hair spray fixative did a good job of binding the charcoal to the paper so brushing acrylic over it didn't just create mud.

Much of the acrylic was applied in thin washes, like watercolor, allowing both the black of the charcoal and the white of the paper to influence the resulting tones.

Charcoal and acrylic on 11 by 14 inch sketch paper.