What is the pink stuff coating fire-ravaged Los Angeles?

Above the roaring fires devastating parts of Los Angeles is an incongruous sight: air tankers dropping gallons of bright red and Barbie-pink slurry over forest, homes, cars, and anything else that might lie in the blazes' path. The substance, vivid against the grey smoke and charred landscape, is fire retardant -- much of it a product called Phos-Chek that has been used by the US Forest Service since the 1960s.