Nissan's TPMS Sensors Color Codes?

Does anyone know why Nissan has different colored TPMS sensors? Here's some examples.

http://thumbs4.ebaystatic.com/d/l225...baHA9zlCag.jpg

https://encrypted-tbn0.gstatic.com/i...4ghQopEFQknFD8

https://encrypted-tbn3.gstatic.com/i...WqMLoo7k9i7pWA

https://encrypted-tbn0.gstatic.com/i...uEdwGUHH-3b0dw

The casing is just a different color. They all operate the same and there are not any differences between the sensors.