Blue roses mean unattainable love and basically just fantasy and red roses are real love and passion for someone.( with a red rose being used in this season it definitely means they will be together for real this time, unlike in the one with the blue rose, which as we know, they didn't have had a happy ending).
Its just common knowledge bit it fits perfectly here! And I'm pretty sure that when he saw the red rose, he realized that in the past when kihoon gave him blue roses that during that time kihoon already remembered their 1st life and didn't tell Yeon about it and endured the sad memories by himself just like in their 1st life together. (This is their 2nd life, Kihoon only lost his memories, its not a new 'timeline' so what Yeon meant was that in both the 1st and 2nd(life before the memory loss) it was Kihoon who had memories of the bad and sad moments and endured that by himself without Yeon knowing.) ╥﹏╥

Can someone explain what yeon meant about the red roses? Or explain the meaning of the blue roses to me again? I already re-read the chapter from when they were kids and the last one from the first season. But I still don’t get it :((