Grand Architect's second ability causes the affected artifact creature to lose any other colors it has, but it doesn't cause it to stop being an artifact.

You can tap any untapped blue creature you control, including one you haven't controlled continuously since the beginning of your most recent turn, to pay the cost of Grand Architect's mana ability. This includes Grand Architect itself.

Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to a blue creature you control may become lethal if Grand Architect leaves the battlefield during that turn.