Yes I used Grease Pencil for the color and shading, I’ll give a breakdown below.
What I do to get the color to fit better is that I have a two pass line art process,
One is just for overall shapes, and the second for all the details.
Both these are on their own layers.
Then I hide the details layer and fill in the basic shapes with the fill bucket, and clean them up where needed with the Sculpt Tool (Grab or Push brush).
When it comes to the Shade I make a new layer higher than the Color layer and set it to Multiply, Opacity around 0.5 depending of the feel you going for and use a dark color, I used Black.
Then I assign a Layer Mask using the Color layer as the mask.
To add the Shade I’ve made a new brush, a pretty simple one.
I made a duplicate of the Pen brush, renamed it to Shapes,
with it selected changed it from stoke to Solid Fill and Pinned it.
Then you can just draw in your shade with shapes on the Shade layer and it should clip to the color mask.
And if you find that you need to mask out parts of your Shade layer,
The best way I found to go about that is to make a layer higher than you Shade layer,
and set its opacity to 0.
Then with your Shade layer selected add another mask higher than the Color mask,
and invert the mask. Then when you draw on the newly made transparent layer it will mask out parts of the Shade layer, this can even be used to get a more Comic Book cross-hatching ink look.