A few of mine are:
Colder than a toilet seat made of brass on the shady part of an iceberg
It’s colder than the underside of a penguin’s b*llsack.
Colder than a room full of ex-wives – Some on here can probably confirm this one
It’s colder than a nun’s thigh. – This one was popular in the catholic school when I was young.
Lets hear some of your favorites.