Luke-
Luke would definitely be that dude who gives you sleepy neck kisses. It'd be just you and him in bed and you'd be his little spoon and he'd just duck in and give you a quick little neck kiss it'd be cute
Michael-
Michael would give you sexual neck kisses and you would always always always know when he wanted YOU because he'd come up behind you and push your hair or clothing out the way and just kiss from your jaw down and find a clear shot in your neck and suck and dude Michael would own you.
Calum-
Calum would give you cute little neck kisses just to show you that he's here for you. Cuddling with him during a movie? Neck kiss. Making food? Neck kiss. Crying? Neck kiss. And it'd just be so subtle but it'd just be his thing okay it'd be sweet as fuck
Ashton-
Ashton would be kinda like Michael with the whole neck kissing but you wouldn't necessarily always have sex because of it I feel like he'd just do it to say he loves you without words a lot. Like you'd just be doing something and he'd kiss up your shoulders to your neck and maybe give you a hickey or maybe just talk into your neck like so you're the only person who can hear him.