Colours I can't answer, but regarding Lethias I think he just doesn't respect his brother very much And sees him as an immature annoyance, but would do anything for him if he were in danger.
Benya still resents Lethias from what happened in the tower back then because before the tower, Lethias was the kind older brother but when push comes to shove, Lethias was really mean to all of them, ALL. To Benya, to Estelle, and most especially, to Sasha. To the Sasha who worked her ass off to make their lives still put together. Benya already likes her that time so seeing how horrible his brother turns out when things aren't going great for them makes him think of him negatively hence he can't fully trust him.
Personally, I think he still loves his brother but it's just that the resentment and mistrust just overpowers his love. Same with his love for Estelle, I think he, too, still haven't grieved for his younger sister coz he knows there is this someone who is close to Estelle who still haven't grieved at her lost hence he sometimes refer to Estelle like she's a stranger rather than his younger sister + remember, in the tower, Benya and Sasha could only trust each other since Lethias became unstable in the tower and Estelle was too sickly.
They're comrades who stuck together with each other, so Benya probably resents Estelle too but not so much since he knew it's not her fault (even when she's sickly and too picky, she still went with Sasha's choices knowing that she's right while Lethias was grumbling here and there).
Anw, this is my answer to that.
I don’t understand what’s going on with the plot anymore like? Also why does her hair color keep changing? Why doesn’t he like Lethias? Or read his letters? Dropping this