In the following tutorials I explain how I generally paint the metallic areas on my models. I use Citadel metallic colours which I find are the best on the market and use basic techniques - shading with washes and adding highlights - as well as a more complicated technique called glazing.
Where this tutorial is plain text with links to fully painted models, my Step-by-Step metal tutorial shows the techniques with high-resolution pictures and less talk.
Painting Silver or Steel:
The metal is given a basecoat of Boltgun Metal and shaded with thinned down Chaos Black. Then I work up highlights from Boltgun Metal over Chainmail finishing with Mithrill. Now the area is subtly glaze down using a mix of purple and dark green ink thinned with water and followed by touching up the final highlights with Mithrill Silver. It is just a question of working back and forth - from light to dark and back again - until it looks right.
Do not be afraid to go bold in the contrasts, just as long as you remember to glaze and re-highlight.
Examples using the technique: My Orc warlord is painted using the technique for pure steel. On my Chaos knights of Nurgle I have used both the steel and gold technique described below in combination with heavy Tamiya glazing and weathering.
I usually start with a basecoat of Citadel Shining Gold. When dry the basecolour is shaded with inks - either brown or purple or a mix (purple if you want your shading to be really deep and with lots of contrast). Hereafter I rehighlight with Shining Gold and then mix Shining Gold with Mithrill Silver for the next highlight. Final highlights are added with pure Mithrill Silver.
Then I start glazing the area down with various orange/yellow/brown/green/etcetera inks mixed with a lot of water and a drop of dish soap. Remember that the glaze needs to be really thin with only a tiny amount of colour in it. I apply multiple glazes until I am happy with the result and I might touch up the final highlights during the process.
A few examples for the gold technique: My Elven noble is painted using basic techniques - gold for basecolour, purple ink for shading highlights with gold followed by mithrill and finally glazed with thinned-down Tamiya Clear Orange. On the IKore Archangel I have used various inks for shading and likewise for glaze, which gives the metal parts a very unique and ancient look.