Gentian Violet is an intensely purple powder. It's used for a variety of non-prescription medical purposes, and at one time was used to make the purple dye they stamped prices on canned goods with at the grocery store. (Anyone old enough to remember that?)
My old boss kept Gentian Violet around because he was an amateur plumber and would flush a teaspoon down the toilet from time to time when he thought he might have a leak, and then go look to see if purple water came up anywhere.
He once put a little bit in his pool, and turned the whole thing purple (much to his wife's dismay.) Gentian Violet is powerfully purple -- get a speck or two on your fingers, and you've got purple fingers for at least a week. My old boss was always coming to work with purple blotches in various places.
One year at Christmastime, he'd go out in the morning to get the paper, and bulbs from the Christmas lights he'd lined the sidewalk with would be missing. He'd replace them, and the next morning there'd be more gone. Gentian Violet, he thought.
That evening, he went out with a popsicle stick and put just a tiny bit of Gentian Violet on each bulb. The next morning he went out to get his paper, and then walked down to the school bus stop. Three little boys stood there with purple fingers, and blotches of purple on their cheeks, noses and foreheads (and Lord knows where else).
The bulbs stopped disappearing. He never ratted them out. Just seeing that sight at the bus stop was entertaining enough.