Participants in this post card swap will use watercolors or acrylics, tea or coffee water, beet juice, etc. and/or texturing medium- anything that can be applied and spread with your fingers. If you use stencils, you must "push" the paint or texturing medium through with your fingers.
No collage, no other elements.
(You may use your fingernails to "draw" paint along, too.)
SAFETY NOTE: I was recently reminded that some paints contain toxic elements like cadmium or cobalt, etc. Safety concerns may be cleared up by checking an art supply site like Dick Blick, for example, to see if there are health considerations associated with particular paints or color pigments. When in doubt, wearing thin rubber gloves like you get at the pharmacy might make it a safer swap technique.
Choose your materials carefully and practice safe-art :)