While they weren't mallards to begin with you can see what others are talking about by misting brown or black over tan.
I recently repainted my floaters, big foots, and shells with this method and am very pleased with the outcome. I use Rustoleum flat white and flat black (in a can for the heads, tails, and cheeks) as well as the Rustoleum flat tan, brown, and black spray paint. I finished the decoys with two coats of Krylon flat clear coat.
Here's what they looked like after pressure washing prior to painting.
Tan base coat.
After shooting the brown over the tan. You can really see the feather detail "pop" after this step.
Finished product with two coats of Krylon flat clear.