Build the color dry first
Use light pressure and short layers so the paper still has room to accept water. A dry first pass makes it easier to control the final strength and keeps small regions from turning muddy. Choose a few related colors, swatch them beside the page, and leave the black outlines untouched.
Activate one small area at a time
Load a small round brush with clean water, blot it once, and pull the pigment across the region. Work from the lightest area toward the darkest edge, rinsing between colors. Too much water can buckle thin printer paper and may cause some inkjet lines to run, so test a spare copy first. Our guide to paper for coloring pages covers sturdier options.
Let layers dry before refining
Once the wash is dry, add a second pencil layer for texture, shadows, or sharper edges. Leave a few highlights uncolored instead of trying to lift pigment later. The free printable library is a good place to test the technique on pages with both large and small regions.