Green makes for good contrast with Spidey's suit, making them recognizable in a scene no matter the angle and distance they're drawn from. Plus, many of those were created when using green, purple and orange for Marvel villains was basically obligatory.
That's the Scorpion for you; looking in the completely wrong direction! lol
But seriously though, this picture is... well, AMAZING! All of the designs look extremely legitimate without making them look "comic book silly," if that makes sense. I would love to see these interpretations in a film! Heck, even in a comic (as long as you were drawing it)!
Oh yeah, something you made me think of with the green villians thing...
Spider Man 3 was originally going to have the Vulture as the 3rd Villian. That would make 2 themes for that movie: a green colored villian theme and a theme about forgiveness. Although Venom is a good take for forgiveness, so is Vulture, because he can steal people's youth in the cartoon.
They should have Saved Venom for his own movie, or the movie being about him being a bad guy, and then he becomes a good guy to take out Carnage.