I assume you are talking about the “Out of Stock” piece I drew. In regard to your question, think about black-and-white pictures. They take a colorful scene and become all different ranges of gray tones.
- Sketch/line your drawing
- Create a new layer and color the drawing ONLY in gray scale. Use as many shades as you please.
- Turn this layer on multiply, and create a new layer beneath it.
- Use this new layer for setting up base colors. (You can hide the gray scale layer for the time being if it makes it easier)
- Once you have both your base colors and shading layer completed (and both are visible) you can begin throwing in some extra colors for definition, smaller details, filters, etc.
I hope this answered your question!