Out, out darn spot!

I have been looking for this all week!   It looks like a regular, white bar of soap, until you get it wet, then the lather turns red.  If you prefer that your victims think they have the plague, and not like they just walked into a scene from Macbeth, there's a bar that turns black instead.

(source here)

"We're going to catch those thieves red-handed."

"What color are their hands now?"