No idea when it changed or how it used to be, but I've only played Rebellion, and I've always been able to colonize when the enemy's culture is about halfway down the phase lane connecting whatever planet to the next one in line. Further than halfway and I usually can't, but if say the culture has just "passed through" the planet and is only about 1/4th of the way down the next phase lane it will expand through, for example, I can colonize there.
