I don't know if CTP was ever really meant to be a sequel and I am sure that Alpha Centauri like Test of Time was meant to sort of finish the idea of the Civ 2 experience...to call them all out sequels...hmmm....Colonization as you speak of is not really a sequel to Civ I...yet, people did call it a sequel...actually, the merchandise packaging actually identifies it as a sequel...hmmm....well, let me assert my opinion....anyone who wants to correct me including Firaxis and Hasbro may....but this is what I really think it the order:
Early 90's "Civilization 1" is born
(A couple years later...demand for more is evident)
"CivNet" is created to expand civ horizons to the networks!
To satify the masses "Colonization" is born.
(6 months later, the masses are still unsatisfied)
by the end of 1995, "Civilization II" is born
(massive hit!)
by the end of 1996, exploration into scenarios is born, along with Modpacks and graphics modifications never before thought of!
Between 1996-1997, "Civ2: Scenarios" (CIC) package is put together to enhance the current civ but not a sequel.
By the end of 1997, the masses have become increasingly restless, so softwares engineers go back to work and....
"Civ2: Fantastic Worlds" is born with massive editing tools, but not a sequel.
The masses are pacified temporarily...but only long enough for their repressed outcry to demand the release of games in 1998 and 1999: "Alpha Centauri, Test of Time, and Call to Power" which were complicated by the purchase of Microprose by Hasbro and the contracting/or licensing of the Civilization idea to Activision. Related games....yes! Sequels...hmmm....Alpha Centauri is in my opinion an "Extension", Test of Time is an "Enhancement", Call to Power was probably meant to be a sequel in its original concept but fell short in so many areas that it just became an enhanced remake of Civ2 through the eyes of another game maker. I've seen the game in action, but haven't played it...perhaps I should to give a fair opinion, but gamers want real sequels and they know when they are being hosed.
Civ III....hmmm....I would definitely say that "it is a sequel" mostly, because there is a commitment to calling it "Civ III". NO ONE seemed commited to that before...I would also suspect that a considerable amount of money and research is being gambled on a loyalty and fanaticism that I can only compare to an early television show that lasted only 1 and a half seasons but has the largest TV show fan club in the world for nearly two decades (star trek).
Well, that's about the best I can tell you...all that just to say your guess is as good as mine, from a guy who hasn't actually played CTP but seen it, so take it for what it's worth. I have played every other civ version except for MGE and I will probably have every version of civ before the end of the year (I'm even considering looking for CivNet, just to complete the collection).
By the way, Colonization is cool, and I have played it extensively...I highly recommend it!
). i kept on waiting for sequal to come out. i would have bet my life it would be alpha centauri. never knew about it. is it a sequal? well, anyway, when colonization came out (why dont they call it col? civilization is to colonization as civ is to col) i played it a lot. great game. awesome economics. all you people who want better economic for civ 3 check out this game! anyway, i think this is as much a sequal as (spat) alpha centauri (spat) so it should have a forum too. 
