In that bloody opening scene, we see the Predator doing what a Predator does - killing, hiding in trees, going invisible... That sort of thing. But then when it's taken down accidentally by McKenna wearing the Gauntlet, there's a big mistake.
We see the invisible figure getting soaked in blood as it lies on the ground, in order to show McKenna what it looks like for the big reveal. But then it wakes up and its eyes aren't even remotely invisible: they're loud, proud and yellow as if the Predator is walking around as a pair of floating yellow eyes when the invisibility mode is turned on.
It's another sign of a film that loses interest in its own story details (and smacks of studio interference because the editing is SO bad). In that context, little errors end up being magnified into giant questions. For instance, if the gauntlet initially locked itself onto McKenna's arm, how did he manage to get it off? How come swallowing the invisibility ball didn't turn McKenna invisible? And how the hell did the Hell Hound escape its imprisonment in the van at the end of the movie?