There are a few "lines" of alcohol inks on the market: Adirondak, Vintaj and SLK Ice Resin inks are my favourites. They can be layered, mixed with glazes or resin or smeared on then distressed. If you have a textured metal surface, alcohol inks really help define the nooks and crannies and give your piece not just colour but depth as well.
I've blogged about this technique before. You don't have to be a great artist to bring a painterly quality to a vintage brass stamping that can become a focal in your work. Apply gesso (found in any art supply store) to the metal. Let it dry. Then take good quality pencil crayons (I recommend using PrismaColor) and build up a thick layer of colour. Brush on a small amount of mineral spirits to blend the pencil crayon and you have a singular focal to coordinate with any design.
Nail polish comes in amazing colours these days and you can brush it onto any metal surface for an instant colour change. Let it dry between coats and apply many layers if you want it more opaque. And just like any metal colouring technique, make sure you use steel wool or sandpaper to rough up the surface a bit and give the colouring agent something to hold onto. Also clean your piece so there are no surface oils or dirt to interfere with a good application of colour.
Heating copper with a torch and quenching it immediately with very cold water will bring out amazing colours in the metal. Reds, oranges, purples and sometimes even greens will ripple across the surface in an almost painterly effect. And it's so much fun!
Opaque and transparent enamels, which are powdered or ground glass, bring instant, bright colour to metal and your jewellery projects. Combine other inclusions to "paint" on your metal, such as frit, stringers and even beads.
Metal bezels can be filled with resin and embellished with objects embedded in the resin. Resin can be coloured as well to bring yet another dimension to your jewellery. Susan Lenart Kazmer has been doing a lot of this lately. Also check Carmi Cimicata's I Love Resin blog for creative resin ideas.
Gold, silver, bronze and multi-colour leafing is easily applied to metal. Using a brush, paint on sizing for metal leaf. Then, using a clean, dry brush pick up a piece of wafer-thin leaf and lay it gently on the sizing. Gently brush it in place and brush away excess sizing. Instant shine!
JAX and other applied patinas
JAX comes in a variety of colours and finishes to bring exciting colour dimensions beyond dull liver of sulphur patinas to your metal and wirework jewellery.
This is a list of my favourite and/or "go to" ways to colour metal. If you have other ideas to share please do so in the comments section.