Richard Crowley wrote:

The wire colors of traditional thermocouples indicate which
type they are (J, K, etc.) and coincidentally also the "polarity".
They have never followed the electronics tradition of black=
negative, red=positive.

Depends on what country the thermocouples are from.
http://www.omega.com/techref/thermcolorcodes.html

I had to deal with these in a multinational factory--parts from
different parts of Europe, Japan, USA, and a mix of mostly K and J, and
nothing labeled.