I was playing around with Krita, coloring some existing drawings. In Photoshop, I used a filter called BPelt to do quick flat color fills on line art. It worked very well. All you had to do was create some line art with closed loops... that is, you couldn't have any open shapes. Like if you had a person's eye you couldn't leave the ink line open or else it would just fill the eye with the same color as the face.
BPelt would fill with random colors. So then you could easily use the paint bucket tool to click and fill each area with the color you prefer. Then it was just a matter of the fun work of doing highlights and what-not.
In Krita, there is a tool called Colorize Mask. You click it, then you get a mask on a layer and you can just sort of draw color lines where you want color to go. So you want the ork to be red, just draw a red line in the ork's body. A blue line across the sky, a brown line across the adventurer's face, etc. Then when you finish the mask it will fill those areas with those colors.
It's pretty nifty. For flat colors, this is the only way to fly. Believe me when I say there's no fun in carefully "hand coloring" vast areas of a drawing with a totally flat color. Might as well let Krita do it for you. Then you can pump your efforts into highlights, color adjustments, shadows, textures, or whatever.