Perhaps hunt for bigger necks? I've got a half-dozen or so trophies, and I get the feeling that the more stars the neck has, the higher the likelihood that it will drop a trophy.

Could just be confirmation bias, but it seems to me that nearly every time I kill a blue one I get another head. That being said, I don't think I've seen a pink one anywhere but on the wiki.

There is a mechanic wherein some mobs do not have stars within a certain distance from spawn. Deer, for example, will have zero stars if they spawned less than a kilometer from the stone circle we spawn in. I have no idea whether a similar mechanic exists for necks, but you might try simply ranging farther afield.