Thanks for reading.
Here was what I was thinking: Arthur, Guenevere and Lancelot are the three major characters, so they ought to get the primary colors. Gender distribution is two and one; "warm" and "cool" color distribution is two and one respectively - Guenevere gets the cool primary color, blue. Yellow is more, pardon the rhyme, mellow than red - Arthur gets yellow and Lancelot gets red.
Secondary colors go to Merlin, Morgan and Nimue. Gender distribution happens to be opposite to the primary color characters, so each secondary character was assigned the complement - the color opposite it on the color wheel - to the color of the primary character who, of the three opposite-gender characters among the six, has the least to do with the particular secondary character. Morgan is Arthur's sister and Merlin's apprentice - she gets green, the complement of Lancelot's color, red.* Merlin is master to both Morgan and Nimue - he gets orange, the complement of Guenevere's color, blue. Nimue is Merlin's apprentice and lover, and (in some versions) Lancelot's mother or foster mother (which is how she's Lady of the Lake and he's "du Lac") - she gets violet or purple, the complement of Arthur's color, yellow. So primary characters got primary colors and secondary characters got secondary colors, and males all got warm colors and females all got cool colors.
Supporting characters get variations on these colors depending on their relationship to the leads. Gawaine and his siblings are in a yellow-green because they're Morgan's and Arthur's nephews; their father Lot's in yellow-green, but Morgause is in a dark green because she's more Arthur's enemy than Morgan is. Ban and Leodogrance and their men and their halls wear their offsprings' colors. Arthur's foster family wear yellow or the olive-drab that yellow becomes when you mix black with it. Pellinore's in a sort of light indigo, because indigo will be Sir Pelleas' color when he takes up with Nimue after Merlin's death, and from their names they must be related.** But that's getting ahead of myself - there are plently of color assignments besides Pelleas' for characters yet to debut (actually Pelleas has had a quick cameo), and you'll learn their colors then.
* Now I think twice, I think I assigned Morgan green first and Lancelot and Arthur their respective primary colors in relation to that, because green is the color of Faerie.
** I balked at giving Pelleas orange after Merlin's death, even though after a fashion he'll be taking Merlin's niche. Or rather, he'll move into Nimue's niche when she moves into Merlin's, but I wasn't willing to give Nimue orange and Pelleas purple either, because that muddled up the primary-secondary character-color relationships. But, I thought, while there are only six colors in the color wheel, those six colors are also in the rainbow, along with a seventh, indigo. Thus, Pelleas' color.
Arthuriana sources I use or recommend:
Arthuriana - the Journal of Arthurian Studies; the website of the quarterly journal of the North American Branch of the International Arthurian Society.
The Camelot Project at the University of Rochester.
Camelot In Four Colors: A Survey of the Arthurian Legend in Comics
Mystical-WWW - The Arthurian A2Z knowledge Bank which has encyclopedically-arranged entries on the characters of the Arthurian legends.
Le Morte Darthur: Sir Thomas Malory's Book of King Arthur and of his Noble Knights of the Round Table, Volume 1 and Volume 2.