Back to the original thread question
One can log IFR time only when the pilot is flying solely by reference to instruments. That does not mean you must be inside clouds. Whenever flying by visual references is not possible, and reference to flight instruments is mandatory for the conduct of the flight, then it is IFR.
Example: you are flying outside of clouds but in less than 2 kilometers visibility, it would be difficult to navigate without any instrumentation.
Example: you are flying at night by a moonless night over the ocean, no outside reference is possible, can not see any horizon, this is IFR time.
Usually I ask myself the question: can I fly now in these conditions by looking outside, conduct a safe flight without being disoriented and putting the aircraft into an unusual attitude. If the answer is no, then this is IFR.
If is independent of your type of flight plan. Now if you are under VFR flight plan into IMC, then you are breaking the rules and that is a different question. You can still log your IFR time though.
Cheers