Colored but not "Painted"
When traveling you need to be sure to have something that is fairly mess free for the kids to do in the back seat and this fits the bill. The only think you need are the pictures, a paintbrush for each kid and a small bit of water.
I searched online for some coloring pages and printed a few out. The fish came from Craft Elf. I printed out the image on card stock since we would be adding water to the picture later and I did not want the paper to rip. When I first started this I did not want anything to detail oriented, just a basic image. I then used watercolor crayons to color the pictures. It is not necessary to completely fill in the pictures as the colors will blend in.
On the picture I colored in I tried two different ways to "paint" the picture. On the back half of the water and the fins of the fish, I used a wet paintbrush to blend in the colors. It worked pretty well as long as I had quite a bit of crayon on the page. On the fish body and the front half of the water I put my finger in a baby wipe and used that to blend the colors. I think it looks a little better but I did run into a problem of the ink from printing the picture bleeding with the baby wipe. This may not be a problem if you are patient and wait a few minutes before you try to color. I have very little patience though and was eager to try it out and complete a few pictures before picking up my son.