Does anyone know where I can look up the color of the leather on my car so I can buy the correct matching leatherique dye?

Use the RPO lookup tool.

http://www.cadillacfaq.com/rpo/index.php

Under the back seat of my 2000 is a large sticker with the leather interior color.

Or in the trunk.

On the inside of the trunk lid under the carpi trim. (I think but I know it's in the trunk)