From a reader:
I think it was Leo Roston who told it this way:
What’s red and wet and hangs on a wall? A herring.
“A herring isn’t red.” It is if you paint it.
“A herring isn’t wet.” It is if the paint hasn’t dried.
“A herring doesn’t hang on a wall.” I just threw that in to make it hard.
(Personally, I think having it whistle is funnier.)