Constructive: "White text on a red background is difficult read, so you might consider a more traditional color scheme, like black on white."
This one is meant to be helpful, so it's constructive.
Non-constructive: "Your stylistic choices would count as a crime in a just world."
This one is hostile and completely unhelpful, so it's non-constructive.
Intent is what matters, though it's possible to be needlessly hostile in a constructive criticism.