You may not know this, but glass is colored with metal oxides rather than with pigment. A typical glassblowing studio will have a furnace of clear glass that you gather glass out of. Color is added after the clear glass is gathered onto your pipe. (To gather : to take glass out of a furnace.)
These are some of forms in which glass color comes in. Shown here are color bar/rod, frit and powder. Frit and powder are the crushed up versions of color bar. Frit comes in a range of grain sizes, all the way down to powder. The great thing about glass color is that you can get a huge variety of effects by changing the method of color application.
Color bar is cut into a desired length, and heated to around 1000 degrees fahrenheit before being picked up out of the kiln and applied to the glass. This process is called a color drop. In most instances, an assistant is required for a color drop.
Frit and powders don't need to be preheated. You can have them ready in a bowl or marver table. You roll your clear glass directly onto the frit or powder, and apply to the surface. You can layer as needed to get the desired color density.