I've used small pieces of broken colored glass by laying the pieces on pots
during the glaze firing. I have used the glass on the outside of pots (face
jugs that provided "ledges" on which to balance the glass) but I was careful to
use small pieces so that the glass would not run to the bottom and stick to the
kiln shelf. I've had good results from this. The color of the melted glass
can be affected by the glaze, but not knowing exactly what you're going to get
is part of the fun.