in your context of makeup enhancing her features he would notice her eyes, the parting of her lips, maybe her neckline if her hair is back. the dress depends on the dress.. he could notice her bust, or the way her hips shift with the fabric, or a majestical grace if she is dressed like cinderella. how he responds depends on the character.. his heart will pause when he notices her, if he is soft like samwell tarly in game of thrones he will freeze completely upon desire and make a fool of himself, if he is stronger he will quickly recover himself and be overly nice to her i.e going out of his way to get her a drink, and maybe having indecent thoughts when her head is turned.. nothing over the top if it isn't a sex book.. just like removing a twig caught in her hair and pausing too long thinking about running a finger along her exposed neckline until she asks if he got it and breaks his train of thought