This small and cryptic poem seems to be about the religious experience of Emily Dickinson's Christianity and the need she feels to "change" and "alter" in response to it. She relates this change to the changes in nature, especially at the close of day and the riotous exuberance of colour that accomanies twilight. Consider how purple is used in this context:
Me, change! Me, alter!
Then I will, when on the Everlasting Hill
A Smaller Purple grows—
At sunset, or a lesser glow
Flickers upon Cordillera—
At Day's superior close!
The reference to a "Smaller Purple" in this poem and the way that it parallels the change that the speaker seems to desire in her own character and person suggests that the purple refers to some kind of divine manifestation of power or glory. In other poems by Dickinson, purple is normally a colour that is associated with the imperial power and person of Jesus, and his robes are often said to be "purple," so perhaps we can relate the usage of purple in this poem to the way that she links purple in with the personage of Jesus in other poems.