This is an awesome art form: painting with live organisms! For this the laboratory techniques of the microbiologist are adapted by artists to "paint" using multicolored bacteria as pigments and a solid agar surface as a canvas. The challenge is that your "brushstrokes" are barely visible initially. To fully see your work you need to allow the bacteria to grow. You can either use natural strains of bacteria, or you can use organisms that have been genetically modified to make them produce the desired colors. The whole procedure is explained in the video below.
Art from the lab of Roger Tsien. Picture by Paul Steinbach. Used under an Attribution-Share Alike 3.0 Unported license.