Simple: i've noticed that apart from first two Predator movies (because the same team worked on, i guess) Predator blood looks different in each movie. I think in AVP it was overglowing and almost white in color. I AVPR and Predators it was opposite: blood almost didn't glow and didn't look very... cool, i think. In The Predator glowing was alright but color seems emerald-like for unknown reasons.
So, is there any satisfactive explanation? Was there any logic put behind it? Also in what movie you think Predator blood looks the best?