Red the flowers, red the crown
Red this day of bleak renown
How soon they forgot Eleanor
Along every oath they swore
Red the flowers, red the wreath
Red the sword that left the sheath
Now a king lies dead on the grass
Taught the vows of princes pass
Red the flowers, red the grave
Red the biers of knights so brave
They who thrice rode and died
Under banners of olden pride
Red the flowers, red the right
Red the fires this day will light
For every slight there is a price
Ours will be long and paid twice.
The red flowers are a reference to the Red Flower Vales, a part of Callow which has frequently been a battleground where Callowans fought against invasions.
Making someone "pay a long price" is a Callowan phrase, which means to ensure that an enemy regrets and suffers for their actions against you long after they happened. As such, the last two lines could be interpreted as swearing to eventually pay back the Procerans by making them suffer twice as much as the Callowans did during the occupation.