Digital Coloring Technique: Tight Line ¤ ¤ ¤
Step One: Okay, first you need to elevate your linework so you can color separate from it. This can be done in a few easy steps. First, create a new layer by clicking the New Layer icon in the Layers palette. Then open to Channels palette (if it's not behind the Layers palette, choose Window>Show Channels). Ctrl+click on the Black channel to load it as a selection. Choose Select>Inverse. Return to the Layers palette and make sure the new layer is the active one. Choose Edit>Fill, and fill the selection with black. Deselect it. Choose Image>Mode>RGB to get your image in the right mode for coloring (don't flatten). Now you've got your inks totally independent from the rest of the image. But you need to get rid if the original inks, so make the background the active layer, and fill it with a solid color. Some people like to color against white, but white is a very strong and bright color. Colors change compared to what their neighbors are, so either choose a background color close to what the final background will be, or a relatively neutral warm or cool gray.
Polygon lasso tool

Active layer &
Preserve transparency check box
 Step Two: Now it's time to start filling base colors. Any layers you add should go beneath the inks layer. You need to figure out what color fill areas will go on what layers. Once you have your color layer segregation organized, you can start making your selections. You can use the magic wand, or the lasso tool for this, but my favorite way to do it is with the polygon lasso. This tool starts you in one place, and creates straight lines between every place you click. To finish, just go to the spot where you started and you'll get a little "o" next to the tool. That means you've found the end and one last click completes the selection. Now make sure you have the appropriate layer for that object selected, and the color you want to fill it with as the foreground color. Choose Edit>Fill. Select Foreground color in the pull-down menu and click okay. Continue on with the rest of the image filling your segregated layers as you go. Make sure you keep everything straight! If you're not sure what's on what layer, Click on the "eye" icon for the layer you want to check in the layers palette. This makes the layer visible or invisible.
  Step three: No you have all your flat colors in, and you're ready to start putting in lights and darks. Be sure that the "preserve transparency" box is checked in the Layers palette for *each* layer. I'm not going to get too in-depth in this process, as Dyno has covered it pretty well in his coloring tutorial, but I will give you some tips.

Big brush with selection


Dodge and Burn tools

A) Choosing colors. When selecting a highlight and a shadow color, choose a highlight color slightly warmer in the spectrum than your base color, and a shadow color slightly cooler in the spectrum. This will add a degree of dimensionally to your image that you can't get any other way.
B) Choose your light source. Decide where your light source is and be sure to stay consistent. Use a secondary light source from another direction if you want, it will add to the realism and make the image more dynamic. Most secondary light sources are typically "reflective" light, rather than "direct" or "indirect" light. Light reflects off objects, (especially light colors or metallics), and carries their colors with it. Try holding a brightly colored object close to a white sheet of paper in strong light and you'll see what I mean. Often when I work on projects I will print the image at this point and do a rough of the lights and darks using a white and a black pencil. This isn't necessary but it gives me a guide to work by and helps me keep the lighting consistent.
C) Matte Vs Glossy. Luminosity and reflective values vary from material to material. Don't use the same amount of highlights on two totally different materials. For example, metallics that are really shiny will be high contrast, where are more matte materials, such as leather, will have highlight and shadow values much closer together.
D) Make specific selections and use BIG brushes. Don't be limited by the size of the area you are coloring, you don't have to find a brush that will fit into it. Try making a selection using the lasso tool that's roughly the shape of the area you want to shade/highlight. Then choose a big brush to create a softer look. You can easily create more brushes if the default ones aren't large enough. Remember that you can also go back in with your base color if you wish to soften selection lines or transitions between highlight and shadow.
E) Be careful with Dodge and Burn tools. It can be tempting to over use these tools when creating highlights and shadows. They can be dangerous because you can't control where in the spectrum they take the colors. I use them, but only to create deep interior shadows and hot spots in highlights.
F) Don't let you're shadows and highlights stay flat. This is a good trick to use to add contrast and depth, expanding on what I said above. Add deeper sections to shadowed areas and hot spots to highlights, using whatever tools you are most comfortable with. This also isn't totally necessary, in some situations with strong indirect light this technique will look strange, but it works under most conditions. This is the key to my soft coloring style, although you could probably adapt it to a cel-style.
G) Don't forget cast shadows. These are really important when creating believable art, so don't leave them out. Remember that the closer the object that is creating the shadow, the deeper and sharper the the shadow will be. Conversely, the further the object is, the softer and more diffused the shadow will be. Cast shadows will follow the form they are cast on, so keep this in mind as well!
  Step Four: When you're finished coloring the image, you have the option to color the inks or leave them black. This is a mostly stylistic thing, it depends on what looks good for the pic and what the artist prefers. To color the inks on your tight linework, check the "Preserve transparency" check box on your inks layer. Then choose a color a few hues darker than the darkest shadow of the area you want to color. Use a hard paintbrush and color the inks by hand. When you come to places where two different colors share the same line, chose the color that overlaps.
  Step Five: Make sure you save at this point. I usually like to keep a version of the file at this point and save future versions under a different filename, but that's totally up to you. So now you're ready to bring it all together. If you want to do a separate background, hide the current background layer and choose Layer>Merge Visible. Now the colored figure is on a single layer by itself and you can manipulate the background or add more layers underneath until you have a background you are happy with. When you're done manipulating the image, choose Layer>Flatten Image. All done! Now you're ready to go back to the Main Tutorial Area.