Right, but I see his confusion...
There seem to be 3 colors of floor liners:
Teak
Very Dark Ash Grey
Black
It is easy to figure out for the Denali Trims:
Light Shale/Teak = Teak Liner
Very Dark Ash Grey/Dark Walnut = Very Dark Ash Grey Liner
Black = Black Liner.
But OP has the Dark Walnut/Slate color combo on the SLT.
I assume that has Dark Walnut Floor, which is a dark chocolate brown -- darker than teak.
Man, bud, not sure exactly how to help you, need to know for sure what color the carpet is...can't believe the dealer can't help you?
I might lean toward the very dark atmosphere, it appears to be dark brown from the pic, which I think would match.
I did notice if I put my VIN in the GMC accessories store, it only presented me w/ the option for Teak Floor Liners...and this matches my interior of Light Shale/Teak. Maybe put your vin in the accessories store, and trust the answer?