Well with a little digging I found on a site called Gamespy several screen-shots of “Colony Island” (images 37-41 if you visit the source website). I have created a slide show (see above) of the more recognizable Colony Island sites. It is not hard to see that these shots are indeed modeled after Roosevelt Island but as with all GTA locations the images take on a much darker and foreboding feel to them. It also appears that President Avenue is the Main Street equivalent in the game and as in reality Colony island is part of Manhattan Isle's counterpart Algonquin Island.
Thanks again to the “blah-blah-blahg” for pointing out what I was curious about in the first place.