Absolutely!
Collards are pretty much a staple for our green smoothies. And I always use turnip greens when available - I'm the "strange guy" at the farmers market asking people for there turnip tops because I have no interest in the bottoms. They are especially abundant now and are a fantastic source of anti-oxidants and Vitamins.
When choosing collards, get smaller leaves that are dark green as they tend to be fresher and milder in flavor.
Give them a try.