Traditionally in the Touraine, and in some other parts of France, horsedrawn carts were painted blue. There are several posited reasons [Source Au temps de Chaumussay by Michel Brouard]:
- the base pigments used for the paint were Prussian blue and barium sulphate, a mixture known as bleu charron. The residue from the manufacture of the blue plant dyestuff guède or pastel (woad/indigo) was also added, and the concoction was an excellent insecticide.
- the blue repulsed flies, which taking the colour for the sky, didn't land on the cart.
- it's the best choice of colour for the job. Red was too agressive, green would mean the vehicles disappeared in the vegetation, yellow was too loud, black was morbid, white was pretentious and got dirty too easily.
I've heard a similar story about kitchens and pantries in the 19th century being painted blue, because it was believed to be a colour that discouraged flies. The colour is known as dipteran blue in English as a result. In France this colour blue is known as bleu charrette (cart blue). Personally I don't believe the colour of the cart would discourage a fly attracted by a cart full of muck. Nor do I believe that woad/indigo would kill wood boring insects (or that carts were in any danger from such creatures). I think the last explanation, prosaic as it sounds, is the most likely.