The sayings I’ve heard most often are…
"Colder than a well digger’s ass." That makes sense, but do they have the coldest asses of all occupations?
"Colder than a witch’s tittie." 
“Colder than a witch’s tit,
Colder than a bucket of penguin shit,
Colder than a hair on a polar bear’s ass
Colder than the frost on a champagne glass”
–Thomas Pynchon, from Gravity’s Rainbow
Regarding witches, I 'd heard: Colder than a witch’s tit in a brass bra.
I’ve always heard it as “Colder than a witch’s titty on a brass broomstick.” Why she’d put her tit on a brass broomstick, I’ve no idea.
Penguin shit?
Penguin shit?
That Pynchon, I always thought he was flying in his own special orbit. Now I know.
The saying I think we should start may not be appropriate for this forum, as it is a tad vulgar. In addition, on the occasions I’ve said it in front of other people (twice), it loses a lot of its punch, since I had to repeat myself both times.
My Grandpa used to say it’s colder than belly-blue hell. Now… I don’t know just how cold belly-blue hell is , but I like the way it sounds . Makes me feel a connection with Grandpa, too . 
So that is my personal favorite.
Cause they’re dirty, at least the hot, female witches are… at least I hope they are. I mean really, what would be the point of being that into nature with perky nipples if you aren’t dirty?
What do you mean by, “Personal Choice” and “Stop objectifying me asshole!”?
My dad often goes for the aliteration: “colder than a Tibetan tin toilet top”.
I’ve always liked "Cold enough to freeze the balls off a brass monkey ".
I think the witch’s tit is a reference to…you know, the witch’s tit the Inquisitors looked for. Where their devil’s familars suckled. I guess that’s pretty cold.
(And can I say I’ve always wanted to see Gravity’s Rainbow filmed. But only as a musical. Have a banana.)
I once worked with a guy who constantly played a really annoying hip-hop CD by a guy who kept proclaiming he was
“Cooler than a polar bear’s toenails …”
(I’m fairly certain it was the same guy who said “If you like fish and grits and all that pimp sh!t, everybody let me hear you say Oh, yah, yer!” So it’s either that I was hearing it wrong, or this guy was more messed up than it seemed.)
In fact, I think it was Wu-tang. Anyone know: are they cooler than a polar bear’s toenails?
Colder than a … really … cold … uh … thing.
… brass toliet seat
… witch’s tit in a brass bra
Brass seems to be a common theme, here.
[Tom Waits]Colder’n a ticket-taker’s smile at the Ivar Theater on a Saturday night.[/Tom Waits]
Polite version: Colder than a witch’s other elbow.
Mrs. Furthur
It’s so cold outside my nipples could cut glass.
Colder than a mother-in-law’s kiss
Cold enough to freeze the balls off a pool table
Colder than moonlight on a tombstone.
Colder than the dark side of an Oort Cloud Object.
Any place cold enough to make your belly blue, specially if you’re actually dressed, sounds pretty cold.
At home we’re boring, we just say “colder than the south pole in winter” or “colder than the heart of a secretary of the treasury”. Half my relatives have worked for the treasury at some point, so we are allowed to make fun of treasury types - they’re “our” guys.
Alert! I’m making this one My Very Own. Thanks.