love's veins...dyed (5): i.e., 'it is blatantly obvious that you have taken your complexion from my lover's veins.'
As T.G. Tucker points out in his edition of the Sonnets: "The word [purple] has the advantage of being used (1) the color of blood in the veins, (2) of the characteristic color of royal 'pride'. Though 'purple' is not 'blue', the adjective (like Latin. Purpureus) might be freely used of bright rich colors; cf. Gray's 'The bloom of young desire and purple light of love (though that line is borrowed from Greek). There may also be the suggestion that the friend's blood was the bluest of sangre azul." (Sonnets of Shakespeare. Cambridge: University Press, 1924, ).
Sonnet 99 is an anomaly in that it has fifteen lines. Some argue that this is not the finished draft, and that Shakespeare would likely have edited it to fit the standard format. However, I believe that Shakespeare intentionally left it containing fifteen lines. In Shakespeare's time, although the majority of sonnets had fourteen lines, it was not unusual for sonnets to contain more lines. And, in this particular case, fifteen lines fit perfectly Shakespeare's poetic purpose. Sonnet 99, with its straightforward list of nature's beauties, is one of Shakespeare's more conventional works. However, if one were looking for interesting elements of the poem to analyze, the disturbing and seemingly out-of-place imagery in lines 12 and 13 would be worthy of that analysis.
Habicht, Werner. Shakespeare's Universe: Renaissance Ideas and Conventions. New York: Scolar Press, 1996.
Martin, Philip J. T. Shakespeare's Sonnets: Self, Love and Art. Cambridge: Cambridge UP, 1972.
Ransom, John Crowe. Shakespeare at Sonnets. Shakespeare, the Sonnets. Ed. Peter Jones. London: Macmillan Press, 1977. 107-8. 1924.
Shakespeare, William. The Works of Shakespeare. Ed. John Dover Wilson. Cambridge: Cambridge UP, 1969.