Frankly, I think all of the actors/actresses in the film
West Side Story
, regardless of ethnicity, color or character, wore make-up.

One must bear in mind, however, that there are different ways for White European Ethnic Americans to look; They range in color from very light/pale-skinned to darker and olive-skinned complexions.

and I think Tony's makeup made him look dark(er) and he was one of the Jets.
I told you a million times not to talk to me when I'm doing my lashes!

I am watching the movie for the first time in ages, and I am struck by how dark Russ Tamblyn's makeup is, as it is for many of the Jets, which I thought odd.

I believe so. It was the cultural norm of the day.

That's very true, leader-7. That was very common in those days, generally, I think.

They did wear makeup to make them more easily identifiable as members of the Sharks. the Sharks also wore darker (and more colorful) clothing. so that when they were in crowded scenes you could more easily differentiate them.

Actually, if one looks at the closing credits, at the end of the movie, under the list of the actors/actresses who played The Sharks and their girls, there are a number of Hispanic names (i.e. Rudy del Campo, Jose De Vegas, to name afew.).
I'm sure that some of the actors/actresses did wear make-up, but that was typical of that day, but even today, actors/actresses who play in movies still wear make-up.