Dust
I always forbade everyone to clean my studios, dust them, not only for fear they would disturb my things, but especially because I always counted on the protection of dust. It's my ally. I always let it settle where it likes. It's like a layer of protection. When there's dust missing here or there, it's because someone has touched my things. I see immediately someone has been there. And it's because I live constantly with dust, in dust, that I prefer to wear gray suits, the only color on which it leaves no trace.

Picasso, from Brassai, Conversations with Picasso

This reminds me of the character in Spike Milligan's novel 'Puckoon' who said something like: 'a good pair of brown shoes won't show the mud, and a good pair of green ones won't show the grass.'