The 1e and 2e monster manuals are oddly lacking the info on teh damage done by the various dragon breath weapons. They list the size of the area effected and that saving reduces damage by half...
Dragon breath does damage equal to the default HP value of the dragon.
I would also appreciate knowing what FRUA does with the chromatic dragons.
Since Tiamat is not part of default FRUA, I can't speak on how FRUA does it, but the 1e Monster Manual says:
"Tiamat's heads are white, black, green, blue, and red. Her breath weapons correspond to the color of each head, exactly duplicating the size and shape of the appropriate dragon's breath weapon and doing damage equal to a huge, ancient dragon of the species applicable, i.e. 56 hit points frost (cold) damage, 64 hit points acid damage, 72 hit points gas (chlorine) damage, 80 hit points electrical (lightning) damage, and 88 hit points fire damage. She can use each breath weapon but once per day. Note that each of her heads is also capable of employing two spells. Tiamat's white head is able to cast two 1st level spells, her black head two 2nd level spells, her green head two 3rd level spells, her blue head two 4th level spells, and her red head two 5th level spells.
"Each of Tiamat's heads can withstand 16 hit points damage before going out of commission until regeneration replaces it the following day."
Each head can also bite, and her tail has a poison sting. FRUA can't duplicate the once-per-day breath weapons, nor the 16-hp-per-head damage, but DC probably could.
