We've read part of the Shadowhunter rhyme in the previous Mortal Instruments books. However, according to Cassandra Clare's new tumblr post, all (the rhyme) is revealed in City of Heavenly Fire...
Black for hunting through the nightCity of Heavenly Fire will be released March 1, 2014.
For death and mourning the color’s white
Gold for a bride in her wedding gown
And red to call enchantment down.
White silk when our bodies burn,
Blue banners when the lost return.
Flame for the birth of a Nephilim,
And to wash away our sins.
Gray for knowledge best untold,
Bone for those who don’t grow old.
Saffron lights the victory march,
Green will mend our broken hearts.
Silver for the demon towers,
And bronze to summon wicked powers.
— Shadowhunter children’s rhyme