I suppose the question also applies to petrified, paralyzed, and stunned as they also inflict incapacitated, but unconscious was the strangest to me.
A creature immune to the incapacitated condition cannot be incapacitated unless something explicitly ignores this immunity
The unconscious condition causes the following effects:
- An unconscious creature is incapacitated, can't move or speak, and is unaware of its surroundings.
- The creature drops whatever it's holding and falls prone.
- The creature automatically fails Strength and Dexterity saving throws.
- Attack rolls against the creature have advantage.
- Any attack that hits the creature is a critical hit if the attacker is within 5 feet of the creature.
If a creature, such as the Warforged Colossus, were immune to the incapacitated condition then only the first bullet point would be prevented. This means the creature would be unconscious, drop what it's holding, fall prone, be unable to move or speak, be unaware of their surroundings, and so forth. This would prevent the creature from doing a great number of things but it notably would not prevent them from taking actions, bonus actions, and reactions, as this is what the incapacitated condition does. Thus, yes, an unconscious Warforged Colossus would be able to take actions as per usual.
However, it is worth pointing out that there are very few ways to apply the unconscious condition to a monster aside from a GM allowing them to make death saves instead of dying outright when reaching zero hit points and a creature choosing to knock a creature unconscious instead of having them die.