Well, depends.
If it is a hot girl with high standards, if you get what I'm saying, saying that she looks like she's blushing or shy may be working as a neg. However, I don't call it a neg, as naturals would say it as well. It's just a teaser which lowers her defence. It doesn't create much attraction, appart for the fact that it's quite cocky to say it perhaps.
It can create a big additional amount of attraction when it is said at moments you had activly increased the sexual tension. Like you kinoed her alot, you're holding her hand or/you're stroking her leg, at the moment you (almost) know that it's on, you can say "are you blushing?/(you look like) you're blushing" to her to build more attraction while looking seducivly in her eyes. The underlying message is "you're exposed. I know you want me".
But litteraly saying this:
Quote:
Omg you know you've gone really red, are you not used to talking to guys?
... is quite offending to my opinion. The only occasions it might work I think is when you say it to a hb9 at a bar, because it's quite rediculous that hb9 would be shy. It's funny also because normaly it's the other way round: guys get shy of her.
But no material to say it to an hb7, especialy not as an opener at shop to someone who works there. Here it's better to say something natural or situational. If you're a nice guy, don't pretend to be a jerk.