From Wikipaedia: TNT [of which nitric acid is a constituent] is poisonous, and skin contact can cause skin irritation, causing the skin to turn a bright yellow-orange color. During the First World War, munition workers who handled the chemical found that their skin turned bright yellow, which resulted in their acquiring the nickname "canary girls" or simply "canaries."
A useless piece of information I learned during a visit to the Royal Gunpowder Mills at Waltham Abbey last week! It promotes itself as 'Secret Island' to attract the kids, but if you get past that (or not!) there's a fascinating amount of history there. Well worth a visit.