Having something perfectly black, like Vantablack level of black, is rather rare as there will typically be some slight hue to it, even if it's really dark. If you've ever worn different black clothing pieces that didn't seem to match, you might've noticed this difference in their hues--one may be slightly more blue while another more brown or purple. This could be something you use and emphasize to create vibration. You can also use the surrounding environment, especially the color of the light source, to add color to it--like the boy's hair in Marco's illustration shown in the video at 3:55. It has color variations both from the blue/green of the forest and sky as well as the warmer yellow-green from the light coming in on the right.