It's fitting that color motifs play such a major role in the novel. After all, color is one of the hallmarks of the movie The Wizard of Oz. Who can forget the moment Dorothy steps out of her black and white world into the Technicolor explosion of Oz?
Movie Oz may be all about a rainbow, but the Oz of Wicked really emphasizes two colors: red and green. And not in a fun, Christmas way either.
Let's start with red. Turtle Heart probably best sums up what this color represents here, with his multiple references to blood, rubies, and the Wizard's red balloon in the first volume.
Red is the color of blood, greed, death, and power run amuck. It is also frequently associated with Nessa's shoes, even though the exact color of the shoes is indeterminate in the novel. The shoes are also described in terms of blood:
The surface of the shoes seemed to pulse with hundreds of reflections and refractions. In the firelight, it was like looking at boiling corpuscles of blood under a magnifying glass. (184.108.40.206)
These shoes are key to Elphaba's downfall in a way; her obsession with them drives her to do some crazy things. And the shoes also give Nessa a dangerous sense of self-importance and independence, which fuels her dictatorship in Munchkinland. Above all else, red is the color of a very dangerous kind of power and violence here.
Now for green, which is a bit more complex. If red is characterized by a series of synonyms (power, blood, greed, violence), then green is associated with a series of opposites.
Green is linked to both Elphaba and to Oz itself. We start hearing about the connections between Oz and green early on in the novel:
In the Oziad, a major mythical and religious text in Oz, we get this description: "Land of green abandon, land of endless leaf." (1.6.14)
We also have a series of important green objects that appear: the Miracle Elixir bottle, the Emerald City, the Clock of the Time Dragon, etc.
What's interesting is that green Elphaba is seen as an abnormality, a freak, an alien in the very "green" land of Oz. In a way, Elphaba is a more "of Oz," with her green skin, than anyone else in the book. And Elphaba is also associated with the green dragon who is dreaming up the world.
But Elphaba is definitely an oddity, someone who doesn't quite belong in Oz. What's interesting is that the Emerald City is depicted in the same way. It's a paradox: the city is sort of the centerpiece of Oz, but it's also seen as out of place and weird. (See Glinda's views on the Emerald City in the "Setting" Section). Elphaba and the Emerald City share an odd sort of symbiosis.
For all her singularity of complexion, it was astonishing how quickly she became camouflaged in the ragamuffin variety of street life in the Emerald City. (220.127.116.11)
It's also worth noting that the Wizard, Elphaba's biological father and the source of her strange hybridity, is most likely Irish, as we learn during Elphaba's dream sequence on (5.11.34). Ireland is of course known as the "Emerald Isle" and is tied to the color green. Green Elphaba got a double dose of the color, from Oz itself and from the "Other World" that she's tied to. Green may ultimately be both a color of belonging and of alienation.