I play Black Templars and Pre-Heresy World Eaters and hate that pristine white look. Here is how I weathered their white.
Do a wash of Bleached Bone (Watered down) let that dry into all the crevaces of the armor.
Using a #2 Pencil add marks to the armor or shoulder pad, using your fingers smudge the pencil marks out and it will look like burns, dirt etc.
Add a wash of brown I use Snakebite Leather (Watered Down)
The trick here is to add the paint to a small ammount of water to get the desired effect of a wash.