Although, there are some early mums out there already...
I personally like to wait on my mums, but they're there, if you need color now. I may buy some now, and pick up some more later.
Also, annual grasses look good this time of year, some Pennisetum rubrum or black millet is very striking.
Asters are starting to bloom, and are very cold-hardy perennials.
I love the look of cabbages and kales in the fall garden. They come in several different colors and forms...not your culinary cabbages, for sure.
You can find several perennials still in bloom, such as rudbeckia and echinacea, and most garden centers have them on sale right now. There's also several which have foliage that turns pretty colors in the fall, like ceratostigma (leadwort), aruncus aesthusifolius (dwarf goatsbeard), and Geranium sanguineum (cranesbill geraniums). And don't forget Heuchera (coral bells), which have foliage in stunning colors year round ( I like 'Dolce Creme Brulee' and 'Caramel' for fall colors, 'Silver Scrolls' for a pretty purple and silver, and 'Amethyst Mist' for a pretty lavender purple).