Path of Exile – What Happens to Characters in Event Leagues?

The item colors are:

• white = normal,
• blue = magic,
• yellow = rare,
• orange = unique
• any of the four colors with border = map with according rarity
• gold = special (orbs, scrolls)
• dark green = active skill gems
• light green = quest items

You can find a list of known vendor recipes here (spoilers!).

It depends on what you are looking for.

When I started playing, all I wanted was scrolls (wisdom and portal), magic and rare items. So I picked up pretty much everything and sold all the items I didn't care about.

Right now, I'm more looking for shards and orbs because I'd like to enhance my gear, and in order to obtain those, I have to follow the famous vendor recipes.
For instance, I need Armourers Scraps and Blacksmith's Whetstons which require superior quality equipment, and also Chromatic Orbs which require items with 3 slots of 3 different colors linked together. In addition of that, I also pick up blue stuff (4 or less slots, like ChrisHateZ said) and yellow/orange stuff when I can find some.

I identify everything before selling my stock because

1. Scrolls of Wisdom can be found super easily
2. You can get more interesting stuff by selling identified items (once again, check the vendor recipes)

So what should you pick up ? You should pick up what you need: powerful gear (blue/yellow/orange) and everything that might be necessary for one of the recipes you are following.