I was told the best crayons or other coloring devices would be to use Crayola crayons. You know, the ones we grew up using. A simple 64 pack would be perfect, but of course that large box would be even better. There are "quilting" crayons/pens available, but they are much more expensive. Plus you do not get the variety you can get using actual crayons.
Be sure to heat set it after coloring. To keep your iron clean, be sure to put a pressing cloth on your ironing board. Mine is just a used towel. I would put a couple of newspaper pages or paper towels under that so the grease from the crayon cannot seep through. Put the design down on top of that with design facing up. Cover that will a couple of paper towels for absorption and another pressing cloth.
For hints on making your design really look great, go to http://www.quiltmaker.com/articles/breath_of_spring__coloring_tips for more information.
If anyone has done something like this before, please share your experiences. We would love to hear what you have done. Send me a picture, too. We would love to see your finished project.
Keep on Quilting