I noticed something. As Willow grows into her magical powers, the color of such energies she manifests changes. Before Buffy was resurrected by her until her descent into darkness, the color of her magical energies was blue. When she became dark, the color was somewhat black. When she returned from her training with Aluwyn, it was green or yellow, depending on the nature of the spells she cast. In Quor'toth, it was green. When she went dark in Quor'toth, it was a somewhat richer shade of orange. When she entered Wonderland and returned to Earth as the only empowered witch in the world, it was yellow. I guess the colors represented the growth that Willow underwent as a witch, but the correspondences of such colors are unclear. But, I guess...
Blue => blossoming
Black/Orange => corruption
Green => greater advancement
Yellow => primal, most powerful
It's just speculation, of course.