In The Giver, Jonas's experiences of the red apple, the faces in the crowd, and Fiona's hair give him a privileged insight into another world, a world that's vibrant, exciting, and, of course, colorful.
Nothing could be further removed from the drab, monotone society in which Jonas currently lives. In the dystopian world that he inhabits, Sameness is the norm, a standard imposed to eliminate conflict.
As the Receiver of Memory, Jonas is able to go beyond Sameness and behold a world of diversity, as he sees manifested in the red apple, the faces in the crowd, and in the change in Fiona's hair.
In relating this experience to the Giver, Jonas becomes frustrated; he can't quite describe exactly what it is that he saw. All he can say, in relation to Fiona's hair, is that it changed. He doesn't know how and he doesn't know why.
It's no accident that all of the objects that Jonas sees in these memories are red. Red is the color of passion, the color of love, two qualities that are notable by their absence from the society in which he lives.
Jonas may not as yet be able to discern the full significance of his memories, but for now, at least, they've shown him that another world is possible—a world characterized by diversity, individuality, and spontaneity.