Emily Dickinson uses the color purple in many of her poems. Regardless, the symbolism of the purple is not necessarily always the same. For example, in her poem "We Like March" (see second link below), the color purple represents the springtime.
In her poem "Wait Till the Majesty of Death", the color purple represents something very different. For this poem's reference to purple, "Full purple in his state", the color represents royalty.
According to a color meaning website,
Purple has been used to symbolize magic and mystery, as well as royalty. Being the combination of red and blue, the warmest and coolest colors, purple is believed to be the ideal color.
Here, Dickinson is referring to the royalty associated with the Majesty of Death. There are other references to Death in which one could justify that Dickinson could be referring to the Lord God given she raises the Majesty to the highest rank possible, "Lord of Lords".
This assumption could be justified by looking at the colors in which God commanded that Moses use the color purple to decorate the linens in a tabernacle. The color purple not only represents royalty, but the highest royalty if one adheres to Christian theology.