States, Colonies, they are still the same people and the same land, so the distinction is just a distraction. Also I find sources that dispute your claim that slavery was banned in Rhode Island.
http://slavenorth.com/rhodeisland.htm
http://en.wikipedia.org/wiki/History_of_Rhode_Island#Slavery
In February 1784 the Legislature passed a compromise measure for gradual emancipation. All children of slaves born after March 1 were to be "apprentices," the girls to become free at 18, the boys at 21. As with other Northern instances of gradual emancipation, this gave slaveowners many years of service to recoup the cost of raising the children.No slaves were emancipated outright. The 1800 census listed 384 slaves, and the number fell gradually to 5 in 1840, after which slaves were no longer counted in the censuses for the state. And, in an essential element of the 1784 compromise, the right of Rhode Island ship-owners to participate in the foreign slave trade was undisturbed.
In 1652, Rhode Island passed the first abolition law in the thirteen colonies, banning African slavery.[13]
It's about halfway down.