Emily Dickinson refers to the color purple in many of her poems ("There is a flower that bees prefer", "Wait Till the Majesty of Death", and "We like March", and "It will be Summer--Eventually").
Typically her use of the color purple lends itself to aspects of nature which many of her poems revolved around. Many times the color purple was the only color used in the poem. The poem "A slash of blue" is very different in this aspect; purple is not the only color used in the poem.
The poem is speaking to Dickinson's appreciation of the morning sky. For Dickinson, the morning sky is a painter's canvas. The colors which are used to depict the morning sky are blue, gray, scarlet, purple, ruby and gold.
For one to understand the use of the color purple in the poem, one must look at the meaning of the color purple.
Purple is the color of good judgment. It is the color of people seeking spiritual fulfillment. It is said if you surround yourself with purple you will have peace of mind. Purple is a good color to use in meditation.
The relevance of this definition is that Dickinson seems to be in search of piece of mind and meditating as the sun enters the sky creating a mixture of many different colors.