Apple blossoms, the flowers containing the eggs that develop into fruit if fertilized, are pink. So, the superficial answer is that the flowers of an apple tree are pink, meaning that if the narrator choose to put apple blossoms in her hair she was choosing to wear pink flowers.
On a deeper level, pink is traditionally interpreted as being the color of eternal love, the color of memories of love lasting forever. The poem, on this level, is a tribute to the love once shared by the narrator and Willie, who is now dead.
Because the narrator "plucked pink blossoms from mine apple tree," she later in the year "found no apples there." The main part of the poem is spent reflecting on the loving pairs of individuals who have been able to gather apples from other trees. She is left with only her memories, which she counts of more worth than the apples gathered by the others.