On a 2015 gmc 6.6 diesel if you use red off road fuel void the warranty on it?

Welcome to the forum.
There may be a mention in your owners manual about Farm fuel.
Personally I would want to contact the dealer and ask the service department, to ensure it is not a problem on your 2015 vehicle.
Give em a call & please & post back they they might say about it!
Now I'm curious too!

I wasn't sure, so I took a look on the interwebbs.

This thread popped up, and may be worth a read, especially the opening post:
The Official Red Dye / Offroad Diesel FAQ - Diesel Place : Chevrolet and GMC Diesel Truck Forums

At one point, according to the link, there was a chance of getting LSD vs ULSD, which is bad for emissions things. While that risk is rather remote now, there is the other concerned raised in that thread, the legal stuff. That can be a big mess, especially if you are not doing all of the work on this truck, because all it takes is a shop to turn you in to ruin your day. Warranty wouldn't be too much of a concern at that point.

Yea, the legality of it would put the warranty to shame.

Basically, don't use it.

Most likely no. Gm can't deny warranty coverage based on farm fuel. I think after 2012 all Manufacturing of Diesel fuel has to be low sulfer. The only difference is dyed fuel is only to determine for off road use. That being said we still have issues with farmers not maintaining their tanks causing contamination of the fuel.

Correction 2006
Ultra-Low Sulfur Diesel (ULSD) Page