Price savings coupon. This is not the sales pitch. That's in the first letter. Only in secretarial school, not in the real world. If your colors match for all pieces of paper, you become boring.
Nothing stands out.
When you're mailing to the automotive aftermarket, use contrasting colors to catch the eye. The most obnoxious color should be reserved for the order form. Orange paper is fine. Please don't make it pale grey. Please don't make it black ink on white.