Green grass turning to a sea of blood, a world full of different grasses, yellow, indigo, blue, orange and rainbow, the legend of a ghost grass from a distant mysterious land.
I wonder if this is a bit of forshadowing to the story as a whole. Westeros is at peace and full of green boys wanting to fight in war. The war of the five kings break up (five colours of grass) and turn the river lands into a feast for crows or a sea of blood? All the while pale as milk glass creatures with an army of the dead amass in a distant mysterious land, ready to destroy all life. @Ghost+Nymeria4Eva, you already pointed out the connection to the others.