Myabe that's why Bumi is a "Squib"? He looks like an Earthbender, but, has no bending abilities, maybe because he doesn't come from Earthbending parents?
Bumi does not look like an Earthbender (i.e. basically Chinese in appearance with green or brown eyes). He has the same gray eyes and complexion as Tenzin, and much the same facial structure:
Basically he looks a lot like Tenzin would look with a full head of hair and a more unruly personality.