Factory incentives are regional in nature, so you'd have to check for your area to see what's available. $28,000 for your truck seems about right for trade in but you should be able to sell it yourself for $4 or $5k more.
Sept and Oct are usually pretty good months to pick up the closing model year. But on a popular vehicle like the Colorado, pickings can be pretty slim. Especially this year.
The other good month is Feb. It's the shortest month of the year, so tougher to meet quota. And it's generally a slow time of year for big ticket items like homes and cars.