Flex fuel gas caps are supposed to be Yellow as far as I know. Have you looked at the build codes to see what it says there? Also, if you are not the original owner it is possible the engine was replaced at one point with a non-flex fuel engine.
With Flex fuel there is supposed to be an extra sensor to detect the alcohol content and some other additional hardware to make it work as well. If that has been removed then you shouldn't be running E85.
With my GF's 2013 Silverado with 5.3 and Flex Fuel setup we found that the drop in Mileage was not worth the cost unless the fuel cost was at least $.40 cheaper to just break even. When a gas station near her work was selling E85 for $1 less per gallon she filled up there all the time but started getting tired of filling her 24 gallon tank all the time and when they increased their E85 to only $.50 less per gallon she decided it wasn't worth it. May be more worth it with the 32 gallon tanks the Avalanches have.
But again, if you are not the original owner you may want to insure the engine hasn't been changed and the flex fuel stuff removed if your vehicle was originally optioned with it.