Yeah that's true, why changin' coulours so fast. I too looked into it, but it might've been that she finally came to know about the torture she was giving Raman just for making him jelly. She might've noticed that she went a little overboard with the Mani jaap. She might've come to know about this in seeing his desperation to say sorry and also wanted to have to say sorry to him, but stepped back when he himself said so.
Raman is a person who doesn't believe everything he sees, but here in this case, when he was going through what he had to suffer years ago was coming back, ofcourse he trusted her at first and believed that Ishita would never cheat on him. Here the thing is not about cheating, it's a bout why are you cheating? because here they haven't confessed yet, there are myriad emotions of the other cheating on them. The first question in his mind was 'Why?' because he knew about their friendship, how close they were, childhood friends.
Ishita would've put herself in his position and thinking form his POV, it is really difficult to handle when one lies to you. And he went through the same thing two times, but luckily she cleared everything before everything was about to fall apart. She realized her mistake and now she's just trying to set things straight.
As for the sudden change, she just did so in fear that whether he would approve of her friendship with Mani, but then he did something completely out of her thoughts. Raman realized that for him stay secure of his relationship with Ishita he has to befriend Mani too. To show Ishita that he too has gone over the petty issues he was on for thinking she was cheating on him and has settled to bring peace among them. Because for him to stay secure at all times he has to also have a good equation with Mani. And my the looks of it, their friendhip is starting out pretty well. I hope to see more of them together.