That doesn't make sense. Why couldn't a region which spans the border between, say, Colombia and Peru have one of those two names? Why not simply select the name whose region is closest to the center of the colonized region?
Further, just because I know where the borders are doesn't mean that gameplay might not lead me to colonize differently. Colonization puts you in competition with other countries, and you'll often not be able to colonize exactly the set of provinces you wanted. Especially in multiplayer, what's to stop players from blocking each other to prevent the assembly of the requisite five provinces? Does that really make for an interesting and compelling game mechanic? It strikes me as arbitrary and unfortunate.
Similarly, if I control, say, Maine, New Hampshire, Vermont and New Brunswick and Nova Scotia, (assuming this constitutes five provineces, three in "Eastern America" and two in "Canada", does it really make sense that picking up, say, Georgia and South Carolina would cause them to merge with my Northeastern provinces to form one colonial nation, where as picking up two other actually adjacent Canadian provinces would do nothing?
I can't think of any situation in which having strictly-defined colonial regions leads to better outcomes than simply detecting contiguous or semi-contiguous regions as the map develops.