I thought it may have been because she mentioned it to him before. Or maybe his color vision is returning due to the power of love. Oh but since the prophecy was kind of "somebody in a world of black and white will stain your would with blood" something like that and red is the color of blood, he can see red?
basically in the novel the mc wrote, the prince reynis gains his sense of color through emotions and the first color he could see was red because of rage (when he found out his mom died) i kinda forgot details but the second color he sees Is pink. The colours he will come to see will have relation with Prim and his growing feeling for her. He sees Peim's blush but didnt know the color name so when she takes him to see the sakura's he could see what color they were and ask her what is the name of the color and she told him pink or something...(it is on wattpad if u wanna read it)
Wait... is no one else wondering how he could tell that it’s red when all he could see before is black and white?