Thermostat O-ring: gray or black?

[Wallace White <wallace@remdesign.com>]

I just received the thermostat and gasket (among other, more expensive
items, naturally!) I ordered from Linda at Carlsen. I ordered the gasket
separately, thinking that the thermostat didn't come with one, but it
did. The one I ordered (N 90136802, the receipt says--I just asked for
the appropriate gasket) is a gray O-ring, while the one that came with
the thermostat is a normal black O-ring (Buna-N, perhaps? just a guess). 

Which one should I put in? Thanks.

- Wallace
  '87 5kcstq 151k