I've been taking some tiny steps toward drawing cartoons. My colored pencils don't work here. They don't produce flat fills. So, I visited Blick's and bought two different types of Faber Castell artist pens. Then, I tried a little experiment.
I filled the circle on the left using one of the Faber-Castell pens. I was not able to achieve a flat fill, although that could be due to my lack of skill with the pens. Notice the brush strokes and differing levels of saturation.
I filled the center circle using my colored pencils. This produced a dappled surface and I found it impossible to completely over-write the white color of the paper showing through. So, no flat surface color here either.
On the right is a vector circle that I filled in Photoshop. Finally, I achieved a uniform flat fill. So, ultimately, using Photoshop for fills of pen sketches is probably the best method for producing flat fills.
Oh, well... I'll continue to experiment. I'm a beginner. It's great to be a beginner at something. Keeps the mind engaged.