When I'm under a tight deadline to get a painting finished, I sometimes don't have the luxury of making "mistakes". Mistakes are usually something I love! They bring about some very rewarding paintings. However, if I'm under the gun and have to produce, I can't afford to make a mistake. So I will make an acrylic skin on glass and lay it down over an area in a painting that I'm thinking about, just to see what it might look like. Especially, as in the most recent painting, when I'm working really large and changing a "mistake" would take a gallon of paint and half a day. So look below and you'll see three different skins I made to check for color against the painting that I had in progress.
Brush the paint out on glass or heavy duty black plastic. Let it dry and then you can peel it up and hold it over the area on the canvas that you are considering. Be sure to make the paint layer thick enough so that you can peel it up and it will hold together. Below I'm holding the dry paint skin over the painted canvas. Looks good!