The hues, gradients, and way lights and shadows fade into one another. You don't get that with coloring pens/markers. Comic book art in the older days had a lot of flat colors (except for the fully hand-painted stuff), and it was up to the pencilers to render the shadows. Nowadays, pencilers tend to ease up on the shadows so the colorists can do most of the lighting effects.
When it comes to Kirby's art, I don't like the way it looks with modern coloring. It was drawn specifically for flat colors to go on top of it.