An effect that changes a permanent's colors overwrites all its old colors unless it specifically says "in addition to its other colors." For example, after Cerulean Wisps resolves, the affected creature will just be blue. It doesn't matter what colors it used to be (even if, for example, it used to be blue and black).

Changing a permanent's color won't change its text. If you turn Wilt-Leaf Liege blue, it will still affect green creatures and white creatures.

Colorless is not a color.

For the second ability, you can choose any single color or any combination of more than one color. You can't choose colorless.

The second ability won't make an artifact creature stop being an artifact. It'll just be a colorful artifact.