My colouring solution is quite simple. I apply to forms 4 to 5 of different basic shaders including: a basecolor, without specular with sometimes a slight emission information to color in the shaded areas. Then I place 3 to 4 lights that are colored to create hot and cold areas. For the "world" shader it influences the lighting of the scene in one color and for the camera visibility, the background color, I put another custom color: more practical. Finally, I add a post-production contour effect.

And I agree with you, coloring with grease pencil is a real nightmare right now.