Faction Relations...

I've seen that to become friends with a certain faction you have to complete some missions for them and they will go green, but it seems that with traders is a little bit different, I've made several missions for traders and they are all still neutral, how can i make them friends?

AFAIK, they can never be friends nor enemies, with some exceptions like Sakharov.

But I played through collector and they were friends from the start of the game, maybe my memory is a little fussy but I remember seeing sidorovich in green, but this just happened when starting the game in collector, and my different playthroughs were from the level 0...

This was at least a year ago, so maybe I'm wrong, I don't know and I'm confused...

You could very well be right. I don't pay attention to trader affiliation as it doesn't seem to affect my game so I've probably been unaware of changes.