S. R. Faruqi:

yak-rangii = love, affection

This theme is entirely new, and in the verse there's also an abundance of meaning. The word yak-rangii , meaning 'affection, love', is very fresh. In the first line 'he has died' also creates the possibility that the roads of love were so harsh that the Nightingale did traverse them, but wasn't able to endure the trials of the journey and yielded up his life. The allusion to the yak-rangii of the flower is also fine, since usually roses are not two-colored.

Then, note that flowers have veins inside them, and if these veins are the footprints of the Nightingale then what it means is that the Nightingale's journey was within the flower. Among the Sufis, the highest level of mystical knowledge is called 'a journey within God'. So if the Nightingale's journey was within the flower, then he was, so to speak, on the level of 'travel within God'.

Another interpretation can also be that the veins of the flower in fact aren't the footprints of the Nightingale, but rather that the hardships and desert-crossings that the Nightingale endured in the journey of passion, and his sincere heart and faithfulness, have had such a deep effect on the heart of the flower that within the flower too, through an affinity with the Nightingale's footprints, lines were created. That is, in the heart of the flower there grew up an empathy for the Nightingale.

It should also be kept in mind that since the veins of the rose are delicate and full of twists and turns, in this respect too they have a similarity with the footprints of the Nightingale.

FWP:

SETS
MOTIFS == ROAD
NAMES == NIGHTINGALE
TERMS == THEME

SRF has done a lovely explication; I have nothing special to add.