When I create a Cycles material I make this process:
1) I take real references. When I make a material I pretend it to be realistic, since Cycles is a physically based render engine. In this case I took these references:
- Tutankhamun mask (Egypt Gold?)
- Yotoco pectoral (Colombia Gold?)
- Some gold ingots
- An ancient coin
You can notice that there are many types of gold materials.
2) I try to find the correct mix of diffuse-glossy-SSS-transparent-etc shaders, making several tests. This was the second material I published in Blend Swap, so I had little experience in Cycles. So I made more than 100 tests. It is very important that you use a good test scene (Tuqueque's Blender Ball or others)
3) Finally I tune or adjust some details, for example colors or textures. In my first attempt I use a color picker. A color picker is a tool which catches the RGB components of a pixel in my reference image. You can use any photo edition software. I use Firefox and its Rainbow addon. Then I make more tests. Depending on my results I pick other pixels or use the blender color wheel to adjust the colors.
Here I show you the Rainbow addon catching a RGB color pointed by the mouse cursor:
TIP: A real material,like gold, has many hues. You can not make it mixing only a diffuse color and a glossy color. I recommend you use several colors and use Fresnel or Facing factors to mix them properly in a physically correct way.
NOTE: I am very proud of this set of gold materials. I used the same nodes to create the copper material. I only changed the colors. It is available in Blend Swap.