I did these with a 3.5" brush - from the paint department of a hardware store - and interior latex paint for the BIG colors. Red, green, magenta, and quinacridone gold are all "real" paint from Golden. I'd bought twelve different intense colors of a good quality interior latex paint, in quarts, eggshell finish, to see what it would be like to use in paintings. It definitely makes me feel freer, less precious about the material. It has a nice consistency - like fluid acrylics - but it does have to be stirred before every painting session.
Anyone trying this themselves, please let me know how it worked for you. If you have a blog or web site where you can post it, include the url in your comment.