Waterbenders are Inuit which explains their darker skin and hairstyles.
The Guru is clearly Indian from his accent and his looks but is the only representation of the Indians in the entire show.
The rest of the world is clearly the typical "asian" that people think of - Chinese, Japanese, Mongolian or Korean. The only other exception are the Swamp Benders, which are clearly from South Louisiana. Freaks.
And for the record, don't lump asians together and pretend that they are all the Sino-Chinese look-a-likes. Asia encompasses the better part of Russia, goes down to Saudi Arabia, all the way to Vietnam and India, the Phillipines, China, Japan, Korea, etc. etc. etc.. That's half the Middle East (the other half is in N. Africa) and the biggest chunk of land ever. That means Jews, Arabs, Indians, over half the Soviet bloc, and all the rest of that stuff are *Asians*.