Metal - Made by Mechanic's Smelt spell, which is always equipped. The spell uses the first non-metal, non-poison mana at the top of your mana stack (farthest left). Spells that affect mana do not affect Metal.
Wind - Made by Monk base spell and used in Combos.
Water - Made as a result of Monk secondary spells or Rain Chant.
Colorless - Many spells will have this icon. It means any type of usable mana (i.e. no poison) will work for the cost.
Poison - Triffids can change your mana into this unusable type. When this happens, it both steals one of your usable mana points, and fills a slot that you could otherwise be using to cast your spells. I recommend pushing the poison mana down the line and out of your mana pool by using spells which generate more usable mana. Getting to a boss with a pool full of poison can seriously reduce your chances of surviving that fight.