When you decide what patterns to paint, here are some other tips (to add to the good advice you've already received):
1. Use a power washer to blast mud and loose paint off decoys. If you're just doing a touch-up, though, you might just want to use soap/water and a soft brush, then rinse very well. The paint will adhere better to a very clean surface.
2. Rustoleum flat oil-based paint, in the quart cans from the home improvement store, comes in a variety of colors, and worked pretty well for me for the large areas. If you want to try flocking them, this makes a pretty good base that is much cheaper than the adhesives that they sell for this purpose.
3. You can get cheapo tubes of oil-based (not acrylic) paint in a set, which you can then mix for your accents.
4. A matte clear coat (from a rattle can) can be applied over the paint to keep the shine down, but be careful! Give it a light dusting from 20" away, let it dry for 15 minutes, and then give it another quick dusting. If you overdo it, it shines even worse!
Here's one I made for a decoy swap out of Styrofoam coated with a 2-part resin, with glass eyes, and fully flocked using Rustoleum.