Purple is a colour that is normally used to symbolise imperial dignity, wealth, standing and stature. For example, in some of Dickinson's poetry, purple is used to describe the colour of Christ's robes, obviously indicating his position as Son of God and saviour of the world. In this short poem, we can see that the colour purple has a similiar symbolism, as if we examine it in context we can see it is related to other aspects of wealth and high stature:
In rags mysterious as these
The shining Courtiers go--
Veiling the purple, and the plumes--
Veiling the ermine so.
Consider the way that the "rags mysterious" veil the "purple, and the plumes" and also the "ermine," which are all objects that we would associate with high class, wealth, and importance. Clearly, Dickinson is creating a comparison by comparing these three aspects of wealth and high standing with the "rags" in which the "shinining Courtiers" are clothed.