Symbolism, Imagery, Allegory
Have you noticed that Camelot seems rather full of weird, mysterious, even heavenly light? So what's up with that, anyway?
Light imagery in the Idylls expresses perfection and otherworldliness. And color and darkness, on the opposite end of the spectrum, represent humanity in its fallen state.
Need an example? Look no further than the good King. Arthur is terribly bright and shiny. Seriously. Not only is he fair-skinned (Bellicent contrasts his fairness “beyond the race of Britons and of men” with her own family’s darkness “well nigh to blackness” [“Coming of Arthur,” 330, 329]), but Guinevere compares him with “the sun in heaven” (“Lancelot and Elaine, 123). She complains that he is practically too bright to look upon and, in “Guinevere,” she says she could not withstand his “pure severity of perfect light” (641). To this she contrasts Lancelot’s “color,” and “touch of earth” (133, 134). While Arthur is all godliness and perfection, Lancelot is a flesh-and-blood, fallible man.
Ah, so that explains it.