Answer: I'm afraid late August would be a little late to start plants from seed for fall color. Remember that in the fall the days are growing shorter, so you need to add a little time onto the seed packets' days to maturity. For example, in the spring, the days are lengthening and it might take a plant 10 weeks to flower, but in the fall it might take 12 weeks or more.
That said, there are a number of plants that will provide nice fall color. Mums, of course, are the classic fall plant, and ornamental cabbages are popular too. Both of these are commonly available as transplants or potted plants in late summer. Many annual and perennial varieties of asters flower in the fall. Pansies love the cooler weather of fall, and will often overwinter to provide early spring color too! Snapdragons and calendula may produce a second flush of bloom in the cooler fall weather. Though they won't withstand a hard freeze, they will endure a light frost. Ornamental chard and kale are durable fall-weather plants, and they come in some interesting colors. They might make a nice foliage backdrop to your brighter flowers.
I'm not aware of any brand new colors for chrysanthemums this fall--though if there are any I bet we'll see them in fall catalogs! If you have a shade of color in particular, I'd be happy to research it for you!
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