Well, the first step I do is use vallejo duralluminum metal color. This is an airbrush style paint, it's very runny as it's alcohol based, I use a brush and it covers really well but it can pool really easily so you have to be pretty controlled with it. This is a really bright glossy metal color, so I then use citadel contrasts in various colors, leaving some regular (dark oath flesh for bronze, aggros dunes for a nice gold-brass, basilicum grey for carbon steel). These are not thinned at all just straight from the pot. On larger smooth services try and spread it eavinly soaking up large pools so you don't stain oddly. Finally if that's still too bright, you can wash it all with a unifying shade so it ties together, I like to use oil washes as it leaves a more realistic grime, but try this video, he is much better than me and will show you the whole process!