The Dr. Ph. Martin's Iridescent Colours flow off a pen tip smoothly and evenly giving an extended writing distance. They dry evenly with no "stop and start" marks. All colours are waterproof, acid free, and Lightfast (non-fading). They are AP approved non-toxic, and conform with ASTM D4236.The colours will flow through an airbrush effortlessly, and are also excellent for Illustration, Designing, and Rubber D4236.Thed in 30ml bottles with the highly preferred dropper top for controlled application. All colours are intermixable with each other.