If you're in shape, like to climb high steep rocky mountains, have the right equipment, have quite a bit of long-distance backpacking experience, and won't mind a lot of scrambling and walking a good mile-an-hour below your typical striding pace, then the Whites in September can be absolutely gorgeous. My best section hike by far.
If those pre-requisites concern you, then there are a lot of other lovely sections to consider. From NYC, you could hike from the Bear Mountain Bridge north to Hanover and start to see the leaves change. Much easier terrain and better suited to a consistent stride. You could head south and go from Harpers Ferry through Shenendoah National Park to Waynesboro, where you won't get the colors in September but it is a lovely time of year and generally warmer. Further south there are even more options, such as Damascus Highlands, Roan Mountain, the Smokies, etc.
I think your issue is how much of a challenge do you want. Best of luck.