If your intent is to paint up the shoulder pads with a chapter specific colour, I suggest going the hipster route and choosing a chapter nobody plays! That's why I'm nominating the [URL="http://wh40k.lexicanum.com/wiki/Metamarines#.UvOe-7uPKUk"]Metamarines[/URL]!
You can't get a sense that this is war being played out on a galactic scale if there are 80,000 Ultramarines running around the place. (Says the Dark Angels player...) So why not show the less seen combats by giving a voice to one of the lesser known chapters?
If you aren't painting them up and going with a black motif then someone suggested Scythes of the Emperor, and I think that would be awesome too!