Thermocouples are classified with letters and are color coded. The most common types are J (black)-, K (yellow)- and T- (blue) type. The wire color indicates what type of thermocouple it is. (example: white + red wire insulations are used for J-type thermocouples; the colors of jacks and plugs for a J-type thermocouple is black.
Thermocouple color codes:
For more information on Thermocouple Color Codes, open this document.