My chair is finally gold!
On Monday I called Rust-Oleum and explained what happened. They suggested that I return the paint to Home Depot. When I accidentally bought the silver paint, Home Depot was out of Rust-Oleum's gold spray paint. They had fully restocked since then, and next to the cans of gold, I realized there was nothing wrong with the cans I bought, their caps just weren't silver (and the cans weren't marked). Luckily Home Depot's customer service department was very understanding. They explained that they couldn't take the silver spray paint back (I'd already used a can and a half), but they gave me two cans of gold spray paint, free of charge.
I love the gold. And the new seat cushion.
To make it, Adam cut a piece of plywood to size for me (1) and I cut a piece of foam to match (2). I used a little spray adhesive to secure the foam to the plywood before wrapping it with some batting (3) and stapling it to the back of the plywood. Finally, I wrapped the whole thing with my fabric and secured it with more staples (4).
I bought the chair on Kijiji a few years ago and the rest of the supplies were leftover from other projects. My only purchases were the fabric and two cans of spray paint, so this makeover was less than $25 - and worth every penny :)