5. On your coloring layer start selecting around the edge of the area to be filled with color.

You can select areas of the
same color all at once.

Be sure to stay on top of the black lines that make up the outline. The selected area is shown in red here just to provide a little clarity.

6. On your coloring layer. Fill the selection with the desired color. This should be the darkest shade you will be using for this color.

This is a view of the color layer
without the artwork visible.

Repeat steps 1-6 for all colors on the entire piece. You will then have a dark coloring book type colored picture.

7. Now select the entire Coloring layer (control+A) Copy the selection to the clipboard. (control+C) Now select the "Channels" tab and create a new channel. Paste your flat colors into this channel (control+V). You will have a grayscale representation of the colored areas. This channel will make selecting similar colored areas easier later on.

8. This is where the fun begins. Go back to your coloring layer and select the area you are going to color with the "Magic Wand" tool set to 0 tolerance. Now select the "Radial Gradient" tool with the settings about what mine are at the right-->. You can experiment with the opacity settings until you are satisfied with the results. Be sure you have your tool set to "Screen" mode. Screen mode is like painting with light it's a bit tricky but a little practice and experimenting and you will get the hang of it.