Yellow buckeye and its hybrids can color up early.

Acer Firefall colors up early for a freeman maple.

Acer Matador colors up late for a freeman maple

Acer Fall Fiesta colors up late and holds late for a Sugar Maple close to that of oaks.

Not sure if you can get away with it but there are birches that color up late around here. Madison and Dakota Pinnacle come to mind.

In my area, Kousa dogwood and American beech color up with the oaks (northern red and white on my property.) The aspens are bright yellow during some of this time, but don't hang on as long as the oak, Kousa and beech.

Many of our red maples color up before sugar maples here, though they do change over a relatively long time depending on moisture (swampy areas turn first) and genetics.

Cornus alternifolia changes with the earliest red maples but I don't know how well it does in your area. Staghorn sumac doesn't quite qualify as a tree, but it turns with the earliest red maples if you have the space for a clump.

Sourwood colors up early, if you can get one to establish. I have had zero luck with two. October Glory usually colors up late, Novemberish, here in NJ. Plus I like the orangey color compared to the other Rubrums.

Ooh great question, Toronado! I am not sure how Atlanta compares to St. Louis, but here, the Willow Oaks get a nice deep yellow later in the season, and their color seems to change slowly and last for a few weeks, more towards when the white oaks change color.

Tulip poplars and sweet gum seem to be on the early side here, September to early October. Tulips always seem to be a pale yellow, and sweet gums can be all types of colors, even on the same leaf. I think the hornbeams tend to be a later color along with the beeches, and those two trees are among my most favorite for fall color (reds to orange to golden to yellow, hornbeam seems to be more red and orange.)

The black gums here are on the very early side, sometimes the first ones to change. Hickories grown in sun have an astounding golden color later in the season, alongside the northern red oaks and pin oaks (which seem to have the muted orange/brown but more brown color, except for shumard oak which seems to have a redder brown color.)

Overcup Oak has nice yellowy-orange leaves, but I forget exactly when they lose them. I want to say they are among the last oaks to get their leaves in spring and the first ones to lose them in the fall, but it has been a while since I read about them. Sassafras has great red colors.

I do not have good color knowledge about dogwoods, black cherries, the lesser discussed oaks (turkey, blackjack, post, chestnut oak, etc), elm, box elder, serviceberry, etc.

There is one of the silver/red maple crosses (color like red, leaf shape like silver) planted at a mall here that changes color really late compared to other maples. I don't know if that is due to its spot being warm (between parking lot and tall masonry building) or genetics, but it is stunning. It could be A. Matador that Whaas mentions.

In response to Ryan, black cherry here turns an insipid yellow for a short time. I've not noticed serviceberry, and since we have a fair amount of it, I doubt that it's really striking since I haven't noticed it having particularly nice color.

Birches seem to overlap all but the earliest turning trees and have a nice clear yellow that hangs on for a fair amount of time. We have paper, gray, yellow and black birches. Native larches turn relatively late, and I love the clear delicate yellow.

Look at Japanese Maples ('Fireglow' in particular). Can you guys grow chinese pistache in zone 6? They tend to have really nice fall color.