In my example the image map shown is the mask for the blending. One color will appear where the image is white, the other when it is black. You have not said whats hooked to the belnding value. 

For a metallic lettering you would need to use a blender node on specular, and probably use alternate specular so you can build a better shader for metals.

This is a common request and you should find some forum threads on the blender node and metallic materials.