Ron heard his name called, and he turned to the sound of it, fully expecting to see his favorite friend in a dark red dress. She'd said she was wearing red, but he hadn't been able to imagaine how she would look; he'd tried to imagine it and had been lost in thoughts as he wondered how differently she'd present herself.
He was not creative enough to visualize this.
He felt stuck for the moment; she was a vision, and he finally felt like he could move forward and did so, shrinking the gap between them.
"Hermione, hi," he said, his eyes sweeping over her dress and then her face and her hair and then back to her eyes, "Hermione," he said again, finding his mouth utterly dry and sticking inside his mouth.
"Wow," was all he could get out as he reached out a hand to her, palm up for her to take, "You look.....just wow," he said again.