Tamara and Geri both wrote in to tell us what she did to make some coloring pages look transparent.
Have the kids color a picture with crayons. They can color dark or light - it doesn't matter but darker tends to look better. Coloring pages with heavy black lines also come out a little better.
...after I left the cooking oil on for awhile it dried fine, so that is what we used. You can make colors transparent with a VERY thin coat of cooking oil. I applied with a paintbrush, and taped to the window to dry. I would not recommend taping to a wall in case it discolors the paint, but its perfect for on windows.