I disagree with the quoted bit:

1) Sokka and Katara are Water tribe, Zuko is Fire, hence the eye colors, which are thus not useful markers;

2) The clothes etc. are very strongly influenced by specific ethnicities (see this enormous screen-shot essay: http://aang-aint-white.livejournal.com/1007.html);

3) re: "vaguely Caucasian features":

http://www.matt-thorn.com/mangagaku/faceoftheother.html
http://shati.livejournal.com/239195.html

In other words: No.

(Edited for accuracy upon re-read.)