To make onion skin paint, you'll need to start by boiling a pot of water. Then, add the onion skins and let them steep for about an hour. After that, remove the onion skins and discard them. Next, add the remaining liquid to a saucepan and boil it until it's reduced by half. Once it's reduced, you can use it as paint or store it in a jar for later use. If you want a darker color, simply boil the liquid down further until it reaches your desired shade.