This excellent poem is a celebration of the arrival of spring, and how the advent of this new season, characterised by new growth and new life, gives the nature that the speaker is looking at an "altered look" of expectation and greater beauty. In particular, this "altered look" is characterised by the way that a "flower is expected everywhere" and the poet imagines that chanticleer walks with "an added strut." The reference to purple that your question refers to comes in line 5, which analyses the way that the "altered look" is characterised by colours. Consider what this line says and the line before it:
The two colours reflect the way that nature is changing in response to the seasons, and capture the beauty of nature as it constantly transforms. I don't personally think that the colour purple here signifies anything beyond a kind of majestic colour that indicates the profound and exquisite beauty of nature. Both the reference to vermilion and purple here serve to heighten nature's beauty and how amazing it is.
A print of a vermilion foot;
A purple finger on the slope...