The Dr. Ph. Martin's Iridescent Colors flow off a pen tip smoothly and evenly giving an extended writing distance. They dry evenly with no ""stop and start"" marks. All colors are waterproof, acid free, and Lightfast (non-fading). They are AP approved non-toxic, and conform with ASTM D4236.The colors will flow through an airbrush effortlessly, and are also excellent for Illustration, Designing, and Rubber Stamping.Sold in 1 oz. bottles with the highly preferred dropper top for controlled application. All colors are intermixable with each other.