I really like learning about all the counter measures and the like that went into these planes. I'm curious about a caption from one of the pictures though. Th

I really like learning about all the counter measures and the like that went into these planes. I'm curious about a caption from one of the pictures though. The yellow diamond of reactive paint on the wing was supposed to give early warning of a gas attack by turning black. Do they mean gas attack like, mustard gas attack or my wing is shot up and leaking fuel?