Incite overwrites all of the targeted creature’s old colors, leaving it just red. It doesn’t matter what colors it used to be (even if, for example, it used to be red and green).

If, during its controller’s declare attackers step, a creature affected by Incite is tapped, is affected by a spell or ability that says it can’t attack, or is affected by “summoning sickness,” then that creature doesn’t attack. If there’s a cost associated with having that creature attack, its controller isn’t forced to pay that cost, so the creature doesn’t have to attack in that case either.

You may target any creature with Incite. It’s okay if that creature is already red. It’s also okay if it would be unable to attack that turn.