Colonization is out so I can talk a bit about it. There are no plans to convert FfH to the Colonization engine, so you won't need to buy Colonization to play FfH.
That said I was a playtester on Colonization and I've been playing it for about the past 9 months. It is well worth checking out, especially if you enjoyed the original (change that, if you enjoyed the original then this is a must buy). It is not Civ4, and Firaxis did a great job of using the Civ4 engine to make a brand new game. Its a lot of fun.
I already suspect we will get requests to implement a few of the things Colonization does so well. Like assigning population to buildings, a more detailed resource model, and being able to seperate population (complete with that citizens skills) from cities as units and take them to join other cities. Basically a focus on your nations economy. The overwhelming responce is going to be that we won't be implementing those systems. Though very cool and perfect for what Colonization is, they aren't what FfH is.