In this article, Darrah MacLean discusses 5 rules for using color in your emails:
Rule #1: Don't overdo it. If there are too many colors competing for attention (or too much of one color), the messaging impact can get lost.
Rule #2: Stay within the palette. While you want each email to feel unique, you also want to be sure to keep the integrity of the brand and not stray from the overall feel.
Rule #3: Use color to improve scannability. Use color to draw the eye across the page, separate sections and make certain areas POP!
Rule #4: Be mindful of color implications. In a retail context, red often means "sale" -- but for other types of messaging, it can signify alarm.
Rule #5: Make sure to keep it balanced. If the main color is green and the supporting colors are orange and blue, go heavier on the green and accent with the others.