Let’s look at a couple of coloring book examples – one for children and one for adults
How would you make a great painting out of the first one? If there was enough shading to make the fish look 3 dimensional and the colors were gorgeous, this would be enough.
What about the adult one? It appears that the line work acts to add values and give a 3 dimensional affect. If you are careful to get the lights and darks correct and use colors based on their values this could work too.
Some people will look at the adult drawing and feel “yipes! too much” and others will think it looks like a lot of fun.