Why Tahno is white:
I’ve noticed many people addressing this issue and asking this question. While it could be that he’s of mixed heritage with an earthbending or firebending and waterbending parent, Tahno could also have parents descending from the South Pole. Some of the characters from the South Pole have lighter skin, like Ummi as pictured above who is Avatar Kuruk’s wife. But again we should remember that genes are an interesting thing and that two parents with dark skin can produce a light skinned child due to a plethora of things, such as a different gene combination.
Just my two cents.