"Automatic sprinklers have various temperature ratings at which their fusible or frangible element (see Coffee Break Training 2008-48) is expected to fail and release water onto a fire. The temperature rating of fusible element automatic sprinklers is stamped on the fusible link. For glass bulb sprinklers, the temperature rating is stamped or cast on a visible part. Color codes also are used for glass bulbs and the frame arms of fusible element sprinklers. In general terms, one must be careful to match the temperature rating of the sprinkler to the maximum expected ceiling temperature to avoid inadvertent discharges. There are many specific rules in National Fire Protection Association (NFPA) 13, 'Standard for the Installation of Automatic Sprinkler Systems' regarding sprinkler placement and temperature rating, so the code official should always check the details carefully."
Coffee Break Training - Fire Protection Series No. FP-2010-8
United States Fire Administration: http://www.usfa.fema.gov/