White
Signifies innocence, as that of a beginning student who has no
previous knowledge of Taekwon-Do.

Yellow
Signifies the Earth from which a plant sprouts and takes root
as Taekwon-Do foundation is being laid.

Green
Signifies the growth as the Taekwon-Do skills begins to develop
like the green of a young plant.

Blue
Signifies the Heaven, towards which the plant matures into a
towering tree as training in Taekwon-Do progresses.

Red
Signifies danger, cautioning the student to excersise control and
warning the opponent to stay away.

Black
Opposite of white, therefore, signifying the maturity and proficiency
in Taekwon-Do. It also indicates the wearer's imperviousness to
darkness and fear.

Taken from "the Encyclopedia of Taekwon-Do" by General Choi Hong Hi.