Peel the outer skins from the onions. The color of the onions will determine the color of your sepia paint. Darker onions will yield a richer sepia color, while lighter onions will result in a more translucent tone. Remove enough skins to fill your pot about half full.
Add enough water to cover the onion skins. Boil on medium-high for approximately 20-30 minutes.
Remove the skins and continue boiling the water until the liquid has reduced. The consistency of the liquid should be thin, but have enough substance to adhere to paper.
Allow the liquid to cool. This liquid can now be used as a paint to change the tone of existing art or combine with other colors to create new works of art.