Colour is what allows the narrator to deepen his experience of the hawthorns. He attempts a few different strategies first, including looking away and back, but to no avail. When the pink hawthorn is pointed out to him he sees it as clearly superior. This shift in colour may have a sexual meaning. White is seen as pure and red as sexually active. Pink may represent the space in between. He is starting to awake sexually he does not catch on to Gilberte's gesture (which-- spoiler alert-- is inviting him to engage in a sexual act).