So, a couple of things to think about.

The medieval Italian city-states that created colonies in the eastern Mediterranean, first in the Levant and later in the Aegean, did not consider their colonies as we might, or even the Romans would have. These were not meant to be places where people lived but commercial enclaves. In the complicated legal world of the Crusader States, long-distance traders needed places where they would be protected by their rights as citizen of a particular polity. What emerged is essentially miniature versions of Cold War Berlin, where you have cities, especially Acre, where Italian cities all had enclaves, usually overseen by the Bishops. Indeed, Genoa was specifically given Parishes in these cities.

Of course, people had to live there. Clergy, innkeepers, and others. The fall of Acre to the Mamluks in the late 13th century exemplifies this. A number of people bearing Venetian citizenship but had never seen the city fled to the lagoons. The city was notorious for guarding its citizenship from outsiders, and these refugees presented a thorny legal and social issue.

As we get to the end of the Middle Ages, we start to see more settlement. Genoa, in fact, offered significant promises of revenue streams from tolls to citizens willing to settle in the Crimea. I suspect unlike the enclaves of the Levant (Constantinople, Alexandria, North Africa, and the Genoese tried to snag an enclave in England [Hull, I believe]), where the Italian cities had a powerful state structure within which they could operate, the Black Sea was a sort of frontier. The Genoese and Venetians, who settled at Tana, needed a ready supply of men to fight rivals (lots of violence between Italians in this era) and, to put it bluntly, you need some muscle to carry on a slaving operation where you dealt with various khans as middlemen.

This brings me to my final point: the development of fundamentally extractive colonies. As the Byzantine Empire weakened, it became increasingly easy for strong military leaders to snag colonies and treat them as feudal demesnes where they could treat the local worse than many peasantries in Europe. Benedetto Zaccaria was given the lordship of Phocaea, where he had access to very valuable alum mines, by the Byzantine Emperor with whom he had a bit of a personal alliance. The Catalan company simply took vast tracts of the the Peloponnese, fortified it, and got rich off the backs of the locals. In essence, these islands represented military outposts and places where people could access the sorts of landed wealth they could never achieve in Italy.

This brings us back to Venice. Venice, with Crete, Cyprus, and Negroponte had in their hands islands that could serve as invaluable military outposts and opportunities to extract wealth. The picture that emerges of the condition of the Greeks who lived there is not, to say the least, a pleasant lifestyle. There is a wonderful article that I would highly recommend finding access as it really describes this question evocatively: Kostas E. Lambrinos, “Identity and socio-economic mobility in Venetian Crete: the evolution of a citizen family,” Mediterranean Historical Review 29, 1 (January 2014): 57–70.

I want to quote from this at some length, but I would highly recommend finding a copy of this article if you require more detail. In the beginning, the author describes how these large islands became opportunities for nobility and landed wealth, not places to transplant Italians.

This elite initially consisted of feudal lords who originated from Venice. Most of them later gained the title of the so-called Venetian nobility (nobiltà veneta), owing to the social transformation brought about by the institution of titles in the Venetian political system (early fourteenth century). The profile of the upper social class continued to change with the appearance of a second-rank title, that of the Cretan nobility (nobiltà cretese), which acquired institutional form in the second half of the fifteenth century (1463), and as time passed, was conferred as a reward for significant services rendered to Venice. At the social level, after 1463 Venetian nobles and Cretan nobles together constituted the dominant class, and played a primary role in public life as members of urban communal councils, from which, as dictated by the Republic's socio-political spirit, the overwhelming majority of the island's population was excluded, as was the case in many Venetian territories. This fundamental political right of the nobles enabled them to conduct collective communication with the central authorities, and to be elected to high-level offices in the local administrative and military machinery.

The article goes on to show how these lords could father children with local Greeks, but they could have a hard time inheriting his title. Locals could earn citizenship, and they staffed many of the administrative posts, but we see the emergence of a second-class citizenry.

It's worth noting that this sort of situation existed in Genoese Corsica and Sardinia, where you have the locals subsisting as share-croppers for much of the Middle Ages.

To conclude, these were not settler colonies. Indeed, I think you see their reflection in the Spanish colonies, where you have plantation owners as a new sort of landed gentry who presided over harsh labor conditions (in this case taken to the extreme of racialized chattel slavery). The idea that colonies could be the location to place a mass of indigent urban labor that could be developed into honest citizens through agricultural toil, which would in turn develop a wasteland wilderness into good, productive land is a uniquely British idea.