Resin

The resin I've used in the past is an epoxy. A resin, hardener and pigments are mixed together and then carefully painted on to a piece. After a day of curing the resin is rock solid and can then be filed and polished. In many ways it's more versatile than enamel. All colours can be mixed together so the palette is almost limitless, this is in contrast to enamels, where the different colours require different firing temperatures and times so mixing colours isn't as easy.

Although the mixing process has to be exact and the application (on the scale that I usually work) can be very fiddly it's generally a much easier technique that enamelling where many things can go wrong in the firing process. With resin colouring if the result isn't what you'd hoped for the piece can be fired and the resin burned away so the process can begin again - with enamel you're stuck with the result unless you have concentrated hydrofluoric acid around (horrible stuff!).