I mean Singapore has 4 official languages, 3 major ethnic groups (Chinese, Malay and Indian), and ~50% of the population are foreigners.

And yes, you a white person can be Singaporean. There aren't many, but there are Singaporean politicians who are white but Singaporean by birth.

I didn't mean legally. I meant culturally. Are they able to be just as culturally connected as everyone else, or would some people assume they are a foreigner even if they were born there?

My experience there was that if you fit within one of the 3 core ethnic groups people would assume your Singaporean. But if you’re in the “other” group then yeah, people would assume you’re a foreigner.

That sounds like HK.

I guess HK and Singapore are multicultural in that you are rather limited in what you can assume is general knowledge.

They don't seem anywhere near as multicultural as places where you hear a wide variety of ethnic groups using the same kind of accent though.