Paint the front of a mat board frame with watercolor, gouache or acrylic paint. Since these paints are chemically stable, there is no chance of the paint reacting negatively with the framed artwork.
Wait for the paint to dry thoroughly before framing the artwork.
Coat mat board with gesso to prepare the surface for oil paints when using the mat board as a canvas for artwork. Allow the first coat to dry, then apply two or more additional coats. If you want to use watercolor, gouache or acrylic paint for your painting, there is no need to prepare the surface with gesso.
Paint your mat board using the same techniques you would use on any other canvas.