How is stress created in glass? How often does the stress cause pieces to break in the kiln?

Stress is caused when the glass cools unevenly.  There can also be stress when different colors are used as they have a slightly different COE.

The glass will not really crack in the kiln if it is annealed properly and the colors have the same COE.  Even though color is a borosilicate, it may not be compatible with other colors, but would be fine in a sculpture made of only that color