The helms pictured have a variety of brass work on them. Almost every part of a helmet can be covered but I prefer to use brass on nasals, ear-pieces, or where multiple metal pieces join.
If it can get brass then that brass can be etched. The possibilities for design are infinite and though I have stuck with classical or Celtic designs many others are possible. It is even possible to have several layers of detail and to use other metals to creat even more detail. So let your imagination go wild.
Blued with aluminum coronet
Price: Blued $25 Coronet $50
I can heat blue armor but the color is not guarenteed. It will vary from a straw to dark blue and though I will try to match your desires this may not always work out. It is also not a rust-proofing but will retard and hide rust better.
The coronets are made of aluminum and though they work best on a great helm they can also work on some other designs though the balls will have to be lower.