The chemistry of a patina is not crystal clear to me, and a lot of the information on the internet is contradictory or difficult to understand. My understanding is as follows:

An oxidising agent such as bleach or hydrogen peroxide decomposes to produce oxygen. Corrosion happens because the metal reacts with oxygen and makes a metal oxide, therefore an oxidising agent accelerates corrosion. Salt also accelerates corrosion because it is ionic and vinegar also speeds it up. Not too sure why vinegar works because a vinegar bath can be used to remove rust, but it seemed to work.

If you're knowledgeable in chemistry perhaps you can come up with a better concoction and can get your hands on some more specialist chemicals (I couldn't even get hydrogen peroxide because apparently it's good for making bombs!). There's probably also an opportunity to create a kind of electrochemical cell using the different corroding metals you can cast and the salt water may act as a salt bridge, but my limited and fading knowledge of chemistry isn't enough to figure that one out. If you do have access to specialist chemicals the following website may be helpful for making more patina colours: http://www.sciencecompany.com/Patina-Formulas-for-Brass-Bronze-and-Copper.aspx.

The following are different methods which I tried:

Bleach, Vinegar and Salt Bath

This worked fairly well for the iron mask and made a reasonably good layer of rust overnight. It smelled appalling so I covered it over and left it outside, as far away as possible. This made both the rusty mask and the brass patina, however it did have some drawbacks. I put the brass mask in a bit longer after the first picture and it didn't go so well, its now quite streaky and uneven, so if you choose to use this method it may be wise to do it for only a short period of time. Also, this made the urethane yellow and slightly sticky. I was expecting this to wear off but after about 3 days there had been no change.

Smelling Salts

I tried enclosing my copper masks in a tub with some smelling salts, using the ammonia released to create the patina. After around 2 hours there had been no change whereas the bath had produced good results in that amount of time. I cannot speculate as to the quality of the patina that would have been produced had I waited, but it would certainly take longer.

Spray-on Bleach, Vinegar and Salt

I found that when spraying on the solution it was more easy for runs, streaks and droplets to form, making the patina a bit less natural and consistent. This was the primary way in which I corroded my copper mask, and I actually really like the results despite it being quite inconsistent.

Bleach, Vinegar and Salt Fumes

I found that the most intense patina was on the parts just outside of the solution. I'm not sure which of the ingredients are actually contributing to the patina (presumably the salt isn't adding anything to the fumes?), but putting a mask in a sealed environment with some of the solution forming vapours produced what I thought was the most natural and rich patina. It also didn't make the urethane sticky, so I think this is the best method of those I've tested.

For my iron powder mask, I found that the rust wasn't really settling in the places I wanted it to, so using some leftover iron powder I mixed it with a small amount of paint and dabbed it on with a paintbrush, then dusted more iron on top so as much bare iron was exposed as possible.

I also used some fine steel wool to rub off the patina in high spots, this compensates for the fact that the mask would have been handled extensively in the time it would have taken to naturally form such a patina. Also, using the steel wool and rubbing in a random motion made lots of tiny scratches in the surface, only noticeable up close but greatly adding to the realism.