What makes some wood age a reddish brown?

  Although I don't weather in the "reddish" genre all that often, I did develop a process for it ions ago. The following description and image is an update of my process:
Step 1: Randomly dry-brush Doctor Ben's Aged Driftwood. 
Step 2: Repeat the process with Doctor Ben's Hardwood Maple. 
Step 3: One more time only with Doctor Ben's Knotty Walnut... Mother nature on the workbench!