Now suppose I activate Celestial Colonnade. What happens? My assumption is that it becomes a 7/7 white and blue Elemental with haste, vigilance and flying until end of turn, and then it goes back to being a colorless 3/3 with haste. Is this correct?
Yes, the land will be 7/7 until end of this turn, and a 3/3 otherwise.
Noyan Dar's and Celestial Colonnade's abilities do the same thing: Turn the Colonnade into a creature that's still a land and set its power and toughness to a specific value. When both effects are active, the latest effect "wins".
Therefore, the order in which you activate them matters. If you first activate and resolve Celestial Colonnade's ability, then trigger Noyan Dar, the Colonnade will be 3/3, because the Colonnade's power/toughness-setting ability will be overwritten by Noyan Dar. If you trigger and resolve Noyan Dar first and then activate the Colonnade, it will be a 7/7.
Noyan Dar's ability has no end condition, and counters always remain on permanents, so the land will be a 0/0 with 3 +1/+1 counters on it when the Colonnade's ability has ended.
Yes, this is correct.
613.6. Within a layer or sublayer, determining which order effects are applied in is usually done using a timestamp system. An effect with an earlier timestamp is applied before an effect with a later timestamp.
(source: Comprehensive Rules)
Since the Celestial Colonnade's has a later timestamp, it 'overwrites' the power and toughness from Noyan Dar. It will still have the +1/+1 counters and Haste; Celestial Colonnade's ability doesn't say it loses those.