If washproof fabric paint is required, Derivan Fabric Fixative mixed with Derivan colours will become permanent on clothes after heat fixing.
Mix equal parts of Fabric Fixative and Derivan colours, then paint and let dry before you heat set with an iron to make the garment wash-fast.
Some synthetic fabrics are non-absorbent and thus not suitable for paint application. Test your fabric beforehand. Also avoid applying the paint too thickly as this may cause the paint to crack.