The sponge tool increases or decreases color saturation at the point, depending on how you set you. If you overcook it, you get the metallic effect.
I know some will disagree with me but personally I don't like the implementation of these darkroom tools (dodge, burn etc.) in photoshop. They're ok for light touchup but I find the layers way more precise, accurate and subtle for real darkroom effects. This is especially true for portraits.