I'll say right off that I am not well read on slavery in Brazil, so I'm going to answer the question in the context of colonial slavery in general. Just keep that in mind

With some exceptions, Romans didn't treat their slaves well at all. That aside, there are differences between colonial slavery and Roman slavery.

Romans were fairly egalitarian in who they took into bondage. Anyone they could defeat in battle was a potential slave and melanin didn't really come into the equation. Rome contained middle eastern slaves, African slaves, Macedonian slaves, Northern European slaves, Balkan slaves and so on. With that being the case, it could be difficult to distinguish a slave from a citizen based solely on their appearance.

Many generations later, it is impossible. Most modern Italians probably have a good deal of "slave blood" in their veins and don't even know it.

Colonialism also played a big role. Roman slaves stayed "in the neighborhood" of their homeland both in terms of geography and culturally. A Roman senator might simultaneously have a Gallic slave and a Gallic wife and a Gallic business partner, for example, which would have some effect on how he treated the Gallic slave. And it would change the slave's perception of himself, knowing that not all Gauls were in bondage.

This was not the case in many colonial societies, where Africans exclusively were slaves. Then skintone becomes an obvious delineation between groups with all the attendant taboos about marriage or social promotion or whatever.

TL;DR: With stars upon thars, slave populations will integrate much more quickly into their larger society.