Darken a procedural wood material quite new to this world, started learning blender with youtube 10 days ago.

So I donwloaded this material :
https://www.blendswap.com/blends/view/73950

I used file>append to add it to my scene from the downloaded .blend file, 
with the node editor I tweaked it so that the grain appear nice on my planks :





but I can't find how to change the color, I'd like it to be darker.
Apparently this is not done in the node editor.

Would anyone know where this is done? 
And while I'm at it, would anyone have a tutorial explaining how those
materials are done?
The only ones I found use an image texture, but If I understood correctly
procedural material do not use textures, everything is generated.

Thanks.
The easiest way is to go to your node editor and add a RGB Curves node inbetween your wood node and the diffuse node (where the circle is in the pic)

Then just bring down the curve. 

You could also TAB into that wood node and look for the colored squares (like your glossy one has) and change the color that way.