I remember how someone pointed out the different color magics of Agatha and her coven of witches that lead me down a comic rabbit hole of magic, but despite not really getting Dottie's roses equaling Dottie being a witch with yellow magic, it did have me stumble on a few details that I think could make sense for thing going forward, even if we didn't get much more explanation from Wandavision's finale.
My current theory is that Agatha's magic is purple because it comes from the Dark Dimension and she channeled Dark Dimension energy through the Darkhold. Agatha having the Darkhold is not only what corrupted her early on, but it gave her access to the "darkest of magics". It also explains why she was able to "steal knowledge" since it was magic she had to study. Also, as you can see, Dormammu has some purple magical energies.
In the comics, we know that Chthon wrote the Darkhold and also that Wanda got her special magical connection due to a link to Chthon. I don't know if that will still hold true in the MCU, but I could see in the MCU that Wanda's Chaos Magic could come from Chthon and would be coded red, while Dark Dimensional energy would be coded purple and Agatha's use of purple magic could be a hint that the Darkhold taps more directly into Dark Dimensional energy.
Now, getting back to Agatha's coven, the magic they had with coded blue, and we see that Billy's magic was as well, so if Wanda's "evil" Chaos Magic came from Chthon, what if the "good witch" magic came from his sister, the elder god Gaea? Gaea is also known as Mother Earth and in the real world Wiccan magic or Pagan Witchcraft is said to be Earth/nature based (probably simplified) so I could see the dichotomy of the good witch magic and the evil magic that the Dark Dimension and Chthon represents.
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In the MCU, it's been established in Doctor Strange that magic comes from dimensional energies, but going forward, I think they might try to separate that a bit. We already know about relics being embued with magic that is too powerful to wield and therefore know about relics like the Staff of the Living Tribunal, which would imply the magic is coming from the Living Tribunal itself rather than a dimension. Also, despite them often referring to "drawing power from the Dark Dimension" it would seem that Kaecilius and the zealots may have been drawing power from Dormammu who is the one said to have immortality and whom they pledge their service to. They could still draw power from the Dark Dimension, but it could explain the diffence between drawing power from dimensions and drawing power from entities and that way Dark Dimension energy isn't just Dormammu's energy, but also energy that Dormammu himself can draw from.
TL;DR
Dark Dimensional energy which Dormammu, Agatha, and the Darkhold uses = purple
"Good witch" energy which comes from Gaea = blue
Chaos Magic which comes from Chthon which Wanda/Scarlet Witch uses = red
