Coloring glass can be an elaborate process with intricate options as well. For surface coloring, it is common to roll the project in powdered colored flux or crushed colored glass. This is a method that lends to giving a single color over an entire surface area. For more focused or small area designs it is possible to use colored rods or sticks to basically draw onto the surface leaving lines of a specific color. Given that in this case, your canvas is molten it does not allow for intricate designs to be drawn directly onto the surface but simple designs are possible.
A lot of chemicals are used in dying glass. Many of the chemicals used could be scary to think about. But given that glass is not porous there is effectively no risk of exposure to the dyes. So when you see Amber glass make mentions of sulfur, black glass make mentions of Manganese, blue glass making mentions of cobalt or green glass making mention of uranium you don’t have to fear it leaching into you or whatever your pipe it used for.
Coloring glass is not just about a case of rubbing color on the surface. Thanks to glassblowing it is possible to mix colored glasses in spirals or swirls to produce a pipe with multiple colors being equally prominent. One more interesting option presented by blowing is the ability to open up a piece and blow another layer of glass inside allowing the artisan to work inside the piece adding intricacy inside of a piece instead of only the outside. This leads into the next subject.