I think that LOTGK should have several dresses, similar to how the White Witch was broken into 7 dress changes. Tilda Swinton's 7 dresses are called (according to The Wardrobe Door, you can check)
the Turkish Delight, Ice Castle, Sleigh, Smoke, Parley, Deep Magic Regalia and Battle Garb. She progressively turns less and less white; her dresses morph and it's quite interesting.
Anyway, I think LOTGK should be the same type, being especially that she is also supposed to be a Northern Witch like Jadis.
First, for when she kidnaps Rilian (This would be the same as when she's a snake and bites Ramda), she should have a light, kind of shining green (lime maybe), like poison
(maybe like this?). I always thought of this dress as thin, a one-piece, kind of because of Lewis's "she was wrapped in a thin green garment". Maybe like Arwen's dream dress? Or else maybe more fitted...I'm not certain.
Second, her next scene would be when she meets Puddleglum and the children during their travels. It should be darker green, not so shiny, but still perhaps kind of flowing.
Maybe like this? And her third scene would be as the Queen of Underland. I'm thinking that there should be several full skirts so that it puffs out in a really intimidating way. Very, very dark green. I'm not sure if it's in the NarniaWeb font colors. When she converts into a snake it could be a darker-colored snake too.