You're likely dealing with dirt and oxidation. I've had great success with a quick brush on of Sodium percarbonate disolved in water, 1 min or so dwell, then rinse with DI or Distilled water. I find that this often restores the color to a very slightly more yellow version of what it would have been originally based on new wood I've used. The sodium Percarbonate is the active ingredient in oxi-clean which is about 70% filler.