You might want to consider buying clipart as well, but free is more fun if you can find what you need. There will be clipart from Spiderman, the newspaper boy, Angels, and anything else you can conjure up. The colors and styles are endless, and only your creativity and the artist will limit you.
The easiest method for colorizing clipart is to use the fill tool. On most graphic editing programs, the fill tool is symbolized by a bucket. Simply click on the clipart you want to colorize, and fill with the color of your choice. This method does work, but may not always yield desired results. If your graphics program offers a magnetic lasso tool, that may be a better option.