Actually, however, when one stops to really think about it, there are different ways that people of different ethnic groups look.
Not all Irish or Polish are light-haired and fair-complexioned. There are people of Irish and Polish descent who are deeper-toned in complexion, and darker-haired as well. Ever head the term "black Irish"? That's where this term comes from.
Not all people of Italian extraction are dark complected and dark-haired, either. There are lighter-skinned and fairer haired people of Italian descent, as well.
Because there are so many different ethnicities here in the United States, and there's a long history of intermarriage between the various white Euro-American groups here in this country, that's where the differences in looks come from.
Hope I've been of some help here.
I also might add, however, that most of the Jets, including Riff, Ice, and Joyboy, had mousy-brown hair and average-looking complexions, if one gets the drift.