I assume, he simply doesn't want to lose his face as the perfect elder brother to his junger brother with dark hair. And is afraid, that minjae will expose him. Also i think he himself maybe is a little insecure and finds it shameful? But I too, dont entirely understand.
In his case, being abused is Jungwoo s normality. He accepted this so seeing another person in same sitution isnt weird for him. And he tried to help Minjae in his own way. By hugging him and helping him to get out of this house.
In my perspective, that hug was his way of comforting Minjae, but his coldness was his numbness to the situation. After that night he pretended everthing that happened didn't happen, it's his way of coping and blocking out the pain. So all he can say was to forget about it and think of it as bad dream. In chapter 45 (I think) it kinda shows that he view himself different from his little brother. This for me gives vibe of "me and minjae are tainted compared to my little brother who is clean and must be protected." So I think that adds to why the older brother is so cold towards him.
Okay yall. There's something I don't understand and needs explaining. The oldest child with the brown hair (jungwoo)... he knows what they went through and yet, it felt like he lack sympathy towards minjae. I get jungwoo perspective of victim blaming himself that he was the reason why this happened to him but this lack of sympathy is getting to me. When they were kids, jungwoo coldly hugged minute as if he didn't care. Why? Even now, he don't seem to sympathize with minjae (if we set aside that jungwoo knows he trying to do something to them). What's the reason for being so cold to minjae when jungwoo knows the pain.