This is a poem that clearly depicts an awe-inspiring scene from nature that Dickinson tries to convey as she watches a sunset silhouette a mountain and make it appear as if the mountain top has a "bloom" because of the way that the sun is setting. Purple is used in the second stanza in the following way:
Seed, had I, my Purple Sowing
Should endow the Day –
Not a Topic of a Twilight –
Show itself away –
Purple is normally a colour that Dickinson uses in her poetry to refer to the divine majesty and imperial rule of God and Jesus, and often purple therefore refers to some kind of magnitude or sense of stunning beauty that can only be related to God's power and creativity. Here, the purple refers to the speaker's act of "Sowing," perhaps refering to the way that she is trying to capture the beauty of this scene, and thus "endow the Day" that she is so struck by. Purple then relates to the majesty of the scene from nature that she is so struck by and which impresses her so greatly.