During Week 4's set of games, there were a few teams that opted to wear throwback jerseys, including the Atlanta Falcons and the Chicago Bears. That led to Jalen Mills to take to Twitter this evening to complain about the Eagles not being able to wear their throwbacks.
The reason the Eagles haven't been able to bring Kelly Green back as an alternate jersey has been the league's one-helmet stance. At the moment the NFL allows only one helmet to be used throughout a season, so the midnight green helmet would be paired with a kelly green jersey, which isn't aesthetically pleasing. Lurie believes he can get a two helmet rule passed at next year's league meetings.
So for now the Kelly Greens will have to stay locked away in the closet, but there's some hope that in 2020 the Eagles will be able to have those uniforms as an alternate.