Go to Michaels or any crafts store and pick up a little bottle of black, raw umber or dark brown colored acrylic water based paint. They are usually about $1 a bottle. As mentioned, fill a cup up with warm water and start adding a little paint to it then rag the color on. Let it get semi or fully dry (pending your preference) and then take a damp rag and wipe off the excess leaving color in the nooks and crannies.
The other way is the same but using a brush to add the color in a thicker manner.
Also as mentioned, test it on something first to get the technique down. The paint can take a color down a notch.