It’s fun painting windows, although it’s a long process. I now have a routine that works for me. White acrylic paint goes on first for all images as a primer. I then apply all the sections of color. It helps save time and limits brush washing by painting all areas of each color one color at a time. Lastly, detail work is added, usually with black. All paint needs to have a few drops of (clear) dish soap added to it to help it stick to the window. With the exception of the primer coat of acrylic paint, I’ve been using Crayola washable tempera because it’s thicker than other brands. The thicker the paint the better. I’m guessing that professional window artists may use a more expensive and window specific type paint, but this works for me.