This is one of Emily Dickinson's poems that concerns the radiant beauty of nature and her attempts to capture what she sees in verse. Often, purple in the poetry of Dickinson relates to the imperial colour of divine rule, and is used to describe, for example, the colour of Christ's robe in heaven. However, in her nature poems it is more often used to refer to the divine majesty of nature and its beauty. Consider how purple is used in this poem:
Some Rainbow--coming from the Fair!
Some Vision of the World Cashmere--
I confidently see!
Or else a Peacock's purple Train
Feather by feather--on the plain
Fritters itself away!
The speaker is clearly struggling to come up with words to describe the radiant beauty of what she is seeing, as the unspecific word "some" clearly indicates. The way that a rainbow is referred to, and then a "Peacock's purple Train" suggests that what the speaker is witnessing is a sunset ("Coming from the Fair"), as the reference to a "Peacock's purple Train" suggests the iridescent splash of colours that accompany such a natural phenomenon. The use of the colour purple here therefore is meant to suggest the beauty of nature.