Does it really matter what the paint color is behind your decoupage? YES if you are after a specific look, the color that you choose to paint your surface does matter. So many times you hear makers and crafters tell you that you have to paint your surface white. Well, you do not HAVE TO paint it white, but if you want to have vibrant colors, then you should definitely choose white or any light color.
The color that you choose will come through your paper - especially when using napkins, tissue, rice paper, or the standard decoupage papers. You can definitely change the tone of your paper, by the color that you choose... or not choose, because you can decoupage onto wood without painting. I will have to show that in another blog.
Here is a perfect example: On the left the background was white (I didn't actually paint it since it was already white) and on the right I painted it using DIY Paint Old School. I decoupaged the same exact napkin on both boxes using DIY Paint Big Top. The one on the right is much more subdued while on the left, the colors are bright and popping.
I have this and several other napkins on my website if you are interested in trying this yourself.