Hi, I wonder if there is a material or paint/dye that would change color when exposed to air? Ideally, it should have the following properties: White-to-black, black-to-white, or any other high contrast color transition (e.g, yellow-to-brown would work too) Works under normal conditions Transition should occur within seconds (e.g., anywhere between 1 and 20 seconds) Color change is irreversible Non-toxic Reasonably cheap Thank you for any tips! P.S. As you can probably tell from my question, I'm totally not an expert in materials or chemistry, so please answer in "plain English". P.P.S. Also, the first thing that comes to mind is how an apple turns brown when one bites off a piece. I think this is kind of the effect I'm looking for.