Most of the symbolism in this novel is quite obvious and unambiguously Arthurian. Since one of the stated saviors of the light is Arthur, the once and future king, much of the symbolism attached to the Arthurian legends is carried over into this story. Things that are quite obviously light (such as Bran, with his name, meaning Raven, which might suggest that he belongs to the dark) are good. Bran is an albino -- he is light-colored, and therefore different than the other children around him, but he is wholly good and part of the light. The symbol of the harp (the ultimate goal of this novel) is one of order, light, and civilization. This is somewhat derived from Arthurian legend, but is even older than that. The harp was a symbol of Apollo, who was the god of light and reason -- and of goodness. The harp of gold represents the order, arts, and the bardic tradition of passed-down tales. All this is in opposition to the chaotic forces of the dark. Though many of the traditional Arthurian symbols are Christian in origin, the use of them in The Grey King is less Christian than it is animistic in its focus.
Generally, in this book, anything that is markedly dark is representative of evil, and the opposite represents good. The whole of the book is about the struggle between the two forces of good and evil.