"The tree has reached maturity and has overcome the darkness...it must now plant seeds for the future."
The color black is created when all the colors of the light spectrum have been absorbed into an object. That object has taken control of the colors and retained them. if one color were to escape, the object would no longer be black but would appear as that color. The student has mastered the nine geup (grades) of taekwondo. He/she has absorbed all the knowledge of the color ranks and overcome or mastered that level or training.
The colors of the spectrum are bound together and are not reflected off an object, resulting in the absence of color in which we call Black.