What does the colour purple refer to in "An altered look about the hills?"

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:

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A print of a vermilion
foot;

A purple finger on the
slope...

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.