According to the manual, there are only three types of orbs that enemies drop. There are definitely green and yellow, I think you may be seeing blue and purple as separate, when they're the same orb (which I will refer to as blue, since that is how my TV renders it).
Health, Focus and Orbs - Ehb is a dangerous place, full of
adversaries who hate the Legion. When an enemy injures you, your
Health will be reduced. If you use your Special Abilities to strike
back, you will deplete your Focus and Power. One way to replenish
these resources is by collecting orbs.
There are Orbs of Health, Focus and Power. Walk over an Orb to
replenish a small amount of the resource associated with that Orb's
You can find these Orbs by defeating enemies and smashing destructible
-- Dungeon Siege III manual, page 5
So, from this and gameplay:
- Green = Health
- Blue = Focus
- Yellow = Power