The furnace in the hotshop contains clear glass, but most glass pieces incorporate color. Color in glass is obtained by adding impurities into the clear glass to produce the color. Cobalt produces blue, Cadmium yellow, Chromium gives green and Sulfur yellow. Fortunately for glass blowers, there are professional companies that have mastered hundreds of formulas for coloring glass. This allows glassblowers to buy their color from these companies. Chemistry degree not required.
Glass from these companies comes in two main forms - frit and bar.
Bar is a rod of solid color about 1" in diameter and about a foot long. It is sold by the kilo, with each bar weighing about one kilo. When you want to use this type of color you break of a chunk of it that's the right size (perhaps 1/10th of the bar). To break off these chunks you can either use a wet saw (my preferred method) or use a color bar breaker. I don't have a picture of one of those at the moment, but it is akin to hitting it with a cold chisel. It can lead to a fairly ugly break. The picture to the right shows my current collection of color waiting for its turn in the hot shop.
Frit, shown in the second picture, is the same density as bar, but it is broken up into sand like particles. It can be purchased in different grades from powder through 00, 0, 1, 2, 3 and 4. The yellow is a zero which is like course sand, and the orange is a #2 which is like small pebbles. Frit is fairly easy to work with as you can roll the hot glass directly onto the frit to pick it up and incorporate it in your work. It doesn't give as even a coating as working with bar, which can be good or bad depending on the looking you're trying to achieve.