Because an individual is photographed "with" a flag doesn't always confirm the subject was a color bearer... as opposed to many that were identified as a color bearer more frequently imaged "holding" the flag...
Further reading down the Auction note updates revealed the following:
AUCTIONEER'S NOTE: It has been brought to our attention that this is a photograph of Nathan Gillette, of Hebron, CT, with the national color of the 1st light Battery Connecticut Vols. This flag still exists and there is an identified photo of Gillette in the State Capitol in Hartford, CT. This appears to be the same photo but printed correctly so that the man's cap is in his right hand and the view of the flag is of its reverse. There is no known reason why the Battery received an infantry sized color, but it did. The large ungainly eagle, executed in shades of gray, appeared on almost all national colors carried by Connecticut troops.
Interestingly the subject is wearing a Cavalry sabre, which in reality is not that unusual for Artillery folks to have been issued.. I have seen quite a few identified Artillery guys photographed sporting Cavalry weaponry than the expected regulation Artillery designated ones...