The black legion have all the cult troops among their ranks, so you could pant the noise marines on their normal scheme (black/pink) and still be 100% fluffy, for other legions, paiting a shoulder pad in the scheme of the legion is a good option. Allows you to paint them on their schemes while retaining come bond with the rest of the army.
At the end you can paint them in any color you like.
Zooming on my plagues I realized that also painted some random parts blue in order to keep them with the theme.
http://www.dakkadakka.com/gallery/161570-Plague%20Marines%202.html?m=2