Of course the obvious would be mums. Also zinnias will still be blooming then (if no early frost) and come in fall colors of red, orange and yellow. I had good luck last year with the common marigolds (rusty orange blend)they were blooming heavily at Halloween. Asters will also be a good choice although they come in bright pink/white/purple (I think the purple would be a pretty contrast with the orange pumpkins) Hope this helps.
Jenn F

I second Zinnias. I grew one last year called "Envy," and I think the lime green would be a beautiful complement to other fall colors!

Things with interesting seed pods might be cool too. I am going to try Nigella for the first time this year, and it looks like it has some very decorative seed pods. I don't know if they'll be around in October, but you might do a little googling and see if they would.

What about Amaranthus? Again, this year is going to be my first time with them so don't know their exact bloom season, but some of them have a lovely drooping habit that would go nicely around the edges of an arrangement, draping over the edges of the pumpkin.

Other late fall bloomers are celosias.
Annual asters were my grandmothers' favorite fall bloomer and she planted them fairly late after spring flowers were done (in her gladiola cutting garden).
The seed pods on Castor beans are extraordinary deep reds and purples and textures that would look good against the smoothness of pumpkins.

Speaking of, how about ornamental gourds, the small ones? That are so neat to look at.

If you go with zinnias, be ready to protect the plants late fall from powdery mildew, to make sure that they keep blooming late.

If you have neighbors who have roses, the late season blooms are often the best of the year, and many rose growers like to share their blooms.

After planning two outdoor weddings for my daughters, one in May and one in October, my suggestion would be to plan an indoor wedding. They both turned out to be beautiful, but both times the weather turned unseasonable cold.

Thank you all for the flower advice. I have ordered some seeds and will see what happens by fall.
DirtyGardener, I have commented about the colder temps to my daughter, but she and her fiance have already booked the venue. Luckily, it has a covered pavillion big enough to host the ceremony if it rains and the venue has large portable heaters should it be too cold.

Somehow the obvious hasn't been mentioned. There is a great chance of a killing frost by October 30. We grew and showed flowers in Chattanooga and Atlanta for years and we sometimes had frost the first week of October, especially the folks in Knoxville and the Ashville area.

Yeah, I would definatly suggest planting a few in containers that can be taken in doors in case a frost comes. That was if by bad luck some get damaged at least you will have those containers one safe.