Purple is a colour that is used frequently in Dickinson's poetry, and normally is used in two principal ways. Firstly, it is often used to describe the colour of Christ's robes and therefore to suggest his imperial majesty and his divine rule. Secondly, it is used in lots of Dickinson's nature poems to describe the rising and the setting of the sun, and the way in which this scene is iridescently beautiful and glorious. We can see from an analysis of this poem that purple is used in the second sense as Dickinson is describing a harbour:
Night after night her purple traffic
Strews the landing with opal bales;
Merchantmen poise upon horizons,
Dip, and vanish with fairy sails.
Purple then here refers to the way that the sunset tinges the various boats and ships that deliver their "opal bales" to the harbour with the glorious purple light that characerises the end of the day. Purple therefore symbolises the beauty of nature.