I use many different techniques to illustrate my books, but for today I wanted to demonstrate one that has its heritage in the great procedural painters of the early renaissance, such as those used by Jan Van Eyck. Now this is photoshop and children's books so the technique is a bit simplified, but if you wanted to you could completely re-create any of the old techniques. So here goes:
You start with a simple black and white drawing of the illustration using varying opacities for your brush. I use a pressure tablet, but you could also just select different opacities by hitting any of the numbers at the top of the keyboard (3 gives you %30, 5 gives you %50, etc.) Use a simple round brush with or without feathering for this.
Make sure that you refine the outline to the best of your ability. Any sort of extra strokes, escpecially white strokes, will show up in the next step. If you are publishing to black and white devices this technique ensures that your illustrations will still be legible on those readers!
Once you have the tone right, create a new layer and set it to Multiply in the Layer Styles pulldown tab. Multiply takes the color you paint and multiplies it by the tone of the color below it. If you use color on color you'll get really vivid dark colors. However, if you use it on black and white you get a flat color with smooth tone. Like this:
There are only four colors used above: red, blue, yellow, and brown. (Also realized that I misspelled Shakespeare here, no matter though, I can go to the previous layer and fix that!) You can mix up the colors and blend them to create more interesting values such as the brown-red in the lower left corner of the book. But this isn't finished, it still needs some tweaking to "feel" like a finished picture.
Add another layer but leave the style normal. This will be your color layer and you will paint directly on top of the other two to polish the image that you've created. Use high opacity color with small brush sizes. This will help create sharp edges and spaces where it feels like more detail then there actually is. When you're done, you should have something like this:
Van Eyck and others used two additional steps to achieve their look. After the black and white layer they would glaze the canvas with raw umber and then apply straight black and white to the "cold" areas of the color. Then they would apply the final color using glazes of oil paint and thinner. The result would be a gorgeous and deep color that would give off the warmth and depth of life. You can read more about this technique here. Interestingly enough, part of the reason the painters developed this technique was to conserve paint, colors such as purple, red, and blue were expensive!
I hope this has been informative. As always if you have any questions about this or anything else relating to your work please say hello on twitter @ajcosmokids