If Celestial Colonnade can't attack unless you pay a cost that includes a mana payment, you may attack with it and tap it to pay for that cost. It will still attack in this case.

When a land becomes a creature, that doesn't count as having a creature enter. The permanent was already on the battlefield; it only changed its types. Abilities that trigger whenever a creature enters won't trigger.

A land that becomes a creature may be affected by "summoning sickness." You can't attack with it or use any of its abilities (including its mana abilities) unless it began your most recent turn on the battlefield under your control. Note that summoning sickness cares about when that permanent came under your control, not when it became a creature.

Once Celestial Colonnade has attacked, tapping it for mana won't remove it from combat.