What brand and model tires are they? Are you flat towing or towing on a dolly? Flat towing shouldn't cause excessive wear, but if the vehicle is not properly loaded on the dolly, it could cause excessive wear on the rear tires.
I would measure the tread depth and then look up what the tread depth is new. The tire manufacturer should list at what depth the tires should be replaced. I've never heard of someone using a color scheme to explain when a tire needs to be replaced. They should have told you or at least wrote down what the tread depth was. I'd ask them to redo it and let me watch them measure the tires.
I wouldn't put too much stock into what the dealer tells you. The last time I had my Xterra in the dealership (about 5 years ago to replace bushings) they told me that my Auto transmission fluid was almost burnt and needed to be replaced. I asked about the Power Steering fluid. I was told it was fine. At the time, I had just drained the tranny fluid and replaced it. It only had about 1000 miles on it, it wouldn't have been burnt. The Power Steering gear box had a leak and the fluid was very low when I dropped it off. Yet they said it was fine. That was the last time any vehicle of ours has been to a dealership.
If your tires are properly rotated, there should not be a big difference in wear. If they aren't rotated then the front should wear faster since the front of the vehicle is heavier.
-Michael
Michael Girardo
2017 Jayco Jayflight Bungalow 40BHQS Destination Trailer
2009 Jayco Greyhawk 31FS Class C Motorhome (previously owned)
2006 Rockwood Roo 233 Hybrid Travel Trailer (previously owned)
1995 Jayco Eagle 12KB pop-up (previously owned)