This is an age old technique which has origins in Europe though India also has an old history of akin to glass is France which is akin to glaze and has been found in the 3rd and 4th century b.c excavations. The staining colours are in fact metallic oxides which is mixed with fluxes and frit and then painted onto glass with a binder. It is then fired in a kiln so that it becomes one with the body. The brushes used are soft long bristle brushes. There are many churches and cathedrals in India which have stained glass treasures which were made in England, Germany, Italy, France etc. and installed in the colonial past.