I’m a florist and the good thing about autumn is you can do any flower color scheme you want. All colors are available. However, what’s in season in Sept is all things orange/bronze/gold/red. Sunflowers are in season. Also rovers, orange lillies or yellow. Carns in burgandy, orange, yellow. Roses come in oranges/browns. Gerbers come in oranges/browns/yellows. Calla lillies come in oranges/yellows. If you like a little blue added to fall color, dark blue delphenium would really perk up an bokay of sunflowers.
There’s also hypericum, which is a berry, which looks excellent in arrangements and comes in red & burgandy. Add a littel salidago (golden rod). Glads are in season in fall, also mums.
The only thing not so much in season would be tulips. I understand crops of Lizzianthus aren’t doing well.
You should consult your own local florist. Be sure and take a budget. Any pictures of bokay styles or colors will be helpful also.