The red stains are from fire retardant cemicals that are pumped into fire fightring airtankers.
(From the 2002 fire season)
"Two C-130 s, equipped with “Modular Airborne FireFighting Systems,” and 60 Airmen have deployed throughout the fire season for an average of 14 days. A MAFFS unit is a pressurized 3,000 gallon tank installed on C-130 cargo aircraft. Retardant or water is dropped out of the tank through two oversized tubes which can establish a fire line in a matter of seconds."