Title:
The title is quite long and is a mouthful but it's unique and I personally like unique titles. The title can kind of hint that there's going to be angst within the story because when you think of balloons leaving people's hands usually it's because a kid accidentally let go and then they're sad 'cause they want their balloon.

Description/Foreword:
I found the whole capslocking and underlining things weird and I even tried to see if you were making a story out of the underlined things but I'm gonna assume not. I feel like the description gave a bit too much away like I feel like the part where he has an illness or whatever could be put in the beginning of the story instead of just out there. I honestly think that the 3 first lines could be the description alone and it might've looked better if it was just those 3 but that's just me.

Plot:
The flashbacks, mainly the second one where they find out that Jungkook has to stay at the hospital, are quite sad in a way. The way Jungkook's parents just kind of give him up like that is sad especially since that does happen even today when a child is sick and they can't afford to keep it. After reading it though, that was really beautiful and the plot was pretty much on point. I love how there was symbolism in the crayon's Seokjin brought him and listening to AKMU's Melted while reading this kind of left a bigger impact since that song is pretty emotional. It's kind of hard to critique something that's pretty much on point.

Writing Style/Flow:
Wasn't expecting it to be in first point of view since the description was in third but the writing style was still beautiful anyways so I'll accept it with zero complaints. The way you described the colors was ing amazing honestly and I can tell you did a little research on some crayon colors (because lemme tell you, I had no idea of any of those colors beside the basic ones). The blue color used for the journal entry was a bit blinding to me so maybe a darker color would have been better.
The changing of POV's was strange since stories should usually be kept in one POV though there are some exceptions to that. I can see why they were needed though and it brought more insight to the story for sure.
The picture in the middle is awkward and should be removed since stories shouldn't have pictures in them if they're not a picture book.

Characterization:
It was a little short to really grasp who they were but we all know that Seokjin is a caring brother to Jungkook and that he did everything he could to make him happy which is sweet. As for Jungkook, you can't help but feel bad because really; who wants to know that they are dying and will go any second? Seems pretty scary to me.

Personal Enjoyment:
I personally enjoyed it and if the title wasn't so long I'd definitely pop it onto the featured stories list (it doesn't fit no matter what font I use, I tried rip) but I do need a new layout anyways so whenever that happens, I'm definitely gonna put it on there. The story is really good and surprisingly, I enjoyed it thoroughly.