I used Testor's Yellow Zinc Chromate for the primer color on the chest pieces and the codpiece. The primer color on the shoulders and knees (since they're already yellow) is more of a light tan color. The Yellow Zinc Chromate really doesn't stand out against the yellow of the armor. I used Krylon "Almond" on my shoulders, and will use it again when I get the knees. It's darn close. You can also custom-mix yellow, tan, and white to get the desired look. 
As far as "carbon scoring" marks go, I used Mars Black acrylic paint, airbrushed, on most everything. I then go over it with charcoal from... MY BBQ GRILL!!! Hey! It's realistic-looking (charcoal is mostly carbon), and you just can't beat the smell of hickory! 
