22 December 2014 Update: I asked Paul Faltyn of the Niagara Aerospace Museum if he could confirm that a color negative of this picture existed. They haven't found one yet but he reported that one of their volunteers, now deceased, had done some colorizing of Bell and Curtiss pictures. He also took the time to check in with a former Bell XS-1 engineer, Bill Swenson, now in his 90s but active. Bill said that the L-39s were olive drab: "He does not recall any of the (Bell) aircraft being painted anything other than green or silver other than the X series, Pin Balls, and a P-59 painted blue for the Navy."
So, are you going to believe Corky Meyer and Bill Swenson or your lying eyes? (Yes, this picture is "flopped" but I wanted to preserve the Niagara Falls Museum legend.)
Corky, who flew both L-39s, had told me, unequivocally, that they were painted "green". For more on the L-39 (and a correctly oriented picture with the blue cast removed) see http://tailspintopics.blogspot.com/2011/04/sweeping-change-bell-l-39.html
Thanks to Ted Smith for following up on the question and bringing this photograph to my attention.
Also, take the time to look at the Niagara Aerospace Museum's excellent Facebook page: https://www.facebook.com/pages/Niagara-Aerospace-Museum