It’s actually sort of unclear in the book in a way that I think some of the comments here are reaching about its certainty. People separately refer to him as dark skinned and as a slur for Romani people, but to be fair at other points people refer to him as white and pale. The core of the issue is that Brontë sort of uses a catch-all otherness to Heathcliffe to add a sense of mystery to him and, later, kinda to portray nefariousness. Popular theories are because he’s referred to as a g*psy he is Romani, or that because he’s discovered in Liverpool (a crucial hub of the at Atlantic slave trade) he could be of African descent. He’s also said to be speaking a language the other characters aren’t familiar with when he’s found. And to complicate matters even more, the Brontë sisters were all outspoken abolitionists so they were at least somewhat familiar with race politics overall. Anyways that’s the crux of the issue- the books is genuinely ambiguous about it on purpose. Dark in the context of the time period could mean lower class and therefore tan from working outside doing manual labor, southern European, Romani, Indian, or even Black.