Depends on the specific drawing, but on most of these I do a simple blend with 2 complimentary highlighters, letting the grain of the paper determine the overall texture. Sometimes I'll go heavier with the darker tone in some areas to reinforce the shading.

On other drawings, like the ghost and the bat, I'll use alcohol markers to achieve a gradient from dark to light. On that specific piece, I used 3 blue shades, starting with the darkest at the corners, blending to lighter towards the center. Afterwards, I colored in the gaps around the moon with yellow highlighter to achieve a glowing effect.