Hal has black hair, Cole has white; Hal's color (indigo) is the opposite of Cole's color (ember, which is a shade of yellow); Hal's personality is witty and sarcastic, and would do anything to accomplish his goals, while Cole is cold and calm, and seeks more honorable means towards his objectives.
Hal's fighting stile evolves around making clean and silent skills, keeping a distance, taking them out one by one with his arrows, or his Shadow Serpents. He often plays dirty. On the other hand, Cole always barges directly in the middle of battle, causing untold havoc, going face-to-face with his opponents, using nothing more than his smoke-enhanced, exploding fists, and his volatile smoke power. He utterly despises dishonor, and will always play fair.
Even their powers are opposing: Shadow is silent, dark and obscure, while Smoke is loud, colorful and painfully visible.
Despite all these differences, the two share a powerful bond, being almost inseparable from one another. They would do anything to help the other, even at the cost of their own lives.