According to Capcom's art director Toshiyuki Kamei, Hakan's turquoise, stone-like hair is a kind of pun. In Japanese, Turquoise is "toruko ishi". While "Ishi" can mean "rock" or "stone", "toruko" can also mean Turkey. This is a nod to the fact that Hakan is Turkish.
Contributed by ZpaceJ0ck0