When I put together my metalX2 WindowBlinds skin, I spent some time looking at images of the metals I needed (found using the image search feature on Google) and then decided which image looked most like the metal I wanted to achieve. I then set about creating gradients by hand, basing the various points in the gradient on the various shades in the image.
The problem with using photographs of a precious metal is that the final result depends on a variety of factors, from lighting to colour reproduction. Many of the images you'll find simply don't look like the metal they represent, or rather how you would perceive that metal to look.
The hardest metals to reproduce are those which don't have a great deal of contrast in shading. Titanium for example just ends up looking like low-contrast grey, with very little to suggest three dimensions.
The most important thing to do is to use references wherever possible though. When you try to create gold from memory, you'll more than likely use a lot of bright yellow and a hint of green. A realistic gold contains neither of these, with the shades ranging from a light peach through a slight purple tinge (yes, purple!) to a dusky orange and finally a deep reddish brown. Likewise, silver isn't just greys, but rather contains hints of blue and bright diffuse highlights. If you just use grey, it'll look grey.
If you need some inspiration, just take a look at metalX2. Just don't steal my hard-earned gradients.
