Now onto the shading. To get the gradients I want, I add the purple ink to the red marker, then apply to the areas I feel the light source hits the least.
The biggest concern here is that you don't add too much of your shading color to your drawing, because once it's added, it can be very hard to take back.
After that, I typically use the darkest color for the eyes
Then I begin cleaning up the drawing, filling in any white spots I can see. Additionally, I add a bit more yellow around the areas where the light source hits the subject, to help them stand out more.
I then add my shading color to the areas I feel the light source won't hit.
Also, I erase any pencil lines that stand out.