Why do the aliens see things as green or red?

From what I saw, the Aliens were reacting purely in self defense. The point of violent contact occurred when the horn was rang by the Human ship... they replied with a louder horn which hurt the Humans. The humans then fired and the aliens returned fire with superior firepower.

Seems like the events escalated into conflict. The aliens 'respect' for human life in also shown in the fact they do not fire unless threatened OR in meeting specific objectives such as cutting off re-enforcement's by destroying the bridge, and the marine base and the mobile police forces. Note however, this military assault began AFTER the battle between the Human ships and the Aliens began at sea.

I just saw the movie last night, despite its poor ratings I liked it a lot.

I think the Red versus Green isn't specifically to do with dangerous items or harmless items, I think its more related to their current objective.

There's probably a default mode that highlights serious weapons like a canon. In the case of the highway, its objective was probably to cut off the potential for any backup to arrive for the humans.

The red and green is to do with wether something is harmless or not but from the scene on the bridge it was merely a necessary target to cut off transportation routes to pave the way for a landing on the island. When Hopper was running away he was still red because they probably already had his intel from the first encounter with him and knew he was a leader along side Captain Nagata if that makes sense so seeing him as a nuisance more then a threat. Also on the scene with the science guy (I dont remember his name) they let him go because he was not threat he was ready to piss himself and they probably did not know what was in his metal brief case because most human tech is considered useless to them except the satalite array. They just focused on the fact that he was ready to piss himself and seen him as no threat and not worth there time.

A) If we were going to make another planet our home, would we kill all the food as soon as we got there?

B) If we went to another planet and found lesser intelligent life and decided to use them for our "menial labor" then we'd try to preserve the ones that are non threatening.

The logic is simple. The color of the objects change according to whether it is harmful or not to the aliens or the aliens want to attack it or not. Now we have to think deeper in this.

The color is "Green" when the object is not harmful in its true sense, meaning if the object is a human being, he should not think evil. We see Hopper had an evil plan to attack the alien in another way. The alien might have perceived it. So he maintained red-alert for him. In case of the cannons, if the direction of them is towards the alien ship, it was shown as "danger" meaning it might fire upon them. The machinegun was never seen by alien ship (I think because the boat was too small to be seen), until it started firing upon the ship. Then they looked at it and it was seen as "red" (as it was firing). In case of pillars, like I said, the alines saw "red", if the object seen is meant to be attacked on. The pillars were meant to be attacked, so it was seen "red".

To question your query to why they did not attack the earth, I would rather say the plan "That they were going to need another planet" was just a mere guessing by Harper. It is not confirmed in the movie. So it might not be true. And that whatever they attacked on earth might be just a precaution, so that the human do not try to attack them.

I'm thinking there was something wrong with their targeting systems. given that they basically couldn't see w/o their systems, they had to rely on them. in the scene where the battleship turns all guns away from the alien ship and the weapons disarm, the alien controller appears frustrated. like he knows its still a threat, but the systems don't and there is poor overrides mechanics.