Everyone and their mom (and my mom), and their friends (and their mom's friends) tells me to design coloring book pages. My answer is typically "I KNOW". At this point I may be late to the game, but it seems like the right thing to do. Of course, there are challenges. Mainly, my work is just too small to color. I've tried. Because I draw with a .005 nib, and most markers or coloring pencils are 5 times that size, it's too difficult to fill in the white space without bleeding into a dozen neighboring lines.
Holding back. Like Coco Chanel said, "before leaving the house, a lady should look in the mirror and remove one accessory." In that same vein, I'm practicing stopping at the halfway point in my coloring-specific drawings. I hold back from filling in leaves with little dots and stripes. I hold back from itty bitty swirls. Hopefully my sacrifice is your successful coloring. :)
Here's my most recent design, hitting the (online) shelves soon: