Could someone please explain why this is imperfective rather than perfective? What would 'nigdy nie użyłem różowego mydła' imply?
That's... difficult. "Nie używałem" would be my first thought - I never used to use pink soap, I just never used something like that.
"Nie użyłem"... sounds as if there was some specific bar of soap and you're telling someone: "I assure you, this bar of soap hasn't been touched, I have never used it!" - so basically, someone assumes that you did. That's at least my impression.