I've got a bunch of annuals and they aren't doing very well. The leaves and flowers are turning brown and purple. Any ideas?
This looks to me like nutrient deficiency; Phosphorus in particular plus a few of the micronutrients. If you've never used compost (decomposed organic matter) now would be a good time. In addition, try fish emulsion for fertilizer. It is a slower release, organic product 5-1-1. Makes plants hardier, more vigorous, more healthier. Then in a week or two give them Osmocote 14-14-14. They won't ever need more (the annuals in your picture, I mean).
Also, when growing annuals, the best thing you can do is cut the flowers off. Annuals have but one job in life and that is to make babies, seed. Once they set seed, it is time to die. The more brutal you are with cutting flowers off, the more vigorous, bud-producing, large, healthy the plant becomes.
I used to do entrance color install and maintenance for homeowner associations. I would cut off every! flower. I had lots of horrified homeowners perplexed. I told them to come back next week, same time. They did and were blown away by the profusion of flowers.
I see seeds on your plants, so part of the problem could be your plants have done their job and they are dying quietly. The fish fertilizer might help, definitely the compost will help. (Evil grin) Cut flowers off! Cut all those seed heads off! All that energy goes back into the plant to use to make itself bigger, stronger and more likely to produce lots of potential seeds. I hope this helps extend your plants' lives.