Red- Power. Typically used by evil characters.
Blue- Unlearned. Or one feels that they are not learned enough to deserve a green blade(ie: Obi-Wan Kenobi).
Green- Mastered. Knights and Masters are usually seen carrying green blades, but it is popular with advanced Padawans as well.
Yellow- Usually associated with healers.
Orange- Honor. Can represent strength as well.
Purple- Uncertainty. Some Masters choose this color to remind themselves that nothing is certain in the Universe.
White- Purity. Typically carried by very light-oriented Jedi.