I like hard cooked eggs as a snack and I store them in the egg carton. To easily tell the difference between raw and cooked, I add some food dye to the cool-down water. I don't aim for anything fancy, just enough of a color to be able to tell which is which at a glance.
Awhile back, I bought some black food coloring to color some caramels (the plan was to make liquorice caramels but I never got around to it. Go ahead and mock me, but I LOVE liquorice caramels.) And today I decided to use the black food coloring.
To the cooling water, I added a glug of vinegar and a squirt of food coloring and let the eggs sit for about a minute or so. I don't think this food coloring was meant to have acid added to it because the dye broke but it resulted in some amazing effects. Some of the eggs have a patina-green color, some look like bronze but the coolest ones look like bronze with blue-green patina. Here is a crappy cell phone photo:
Take that, Martha Stewart!