Just like every other kind of sex there must be consent. In this case the consent must be implied, as even asking permission would be to venture into compliment-paying, so there’s every chance you’ll get it wrong, consent won’t be given and you’ll have overstepped the mark. It’s time to back off again, and this time don’t start pontificating about how modern girls can’t accept compliments. They’re perfectly capable of accepting compliments, but they don’t have to take them from you.