plurality, elements, colors
Kel and Pim are dragons, which means they are creatures of the imagination (or of spirit, if you like), but we like to think that they have some connection to the world of matter
Orpiment, in fact, we think of as mostly matter, her yellow and red color scheme deriving from various antimony compounds, and her fire produced by a blazing stream of organicantimony compounds like trimethylstibine (which are spontaneously flammable in air, you see!)
we've also been tuning her mixture to get different colors of smoke Pim can puff out. white is Sb2O3, we can get pink and red shades with Sb2S2O and Sb2S3, and elemental Sb gets us black
Kel's purple and green colors we haven't really rationalized in this way, though Kel's green _glow_ we attribute to thallium (the bright 535 nm emission line that Kel loves so much.) the pairing of purple and green can be found in the related arsenate minerals erythrite and annabergite, "cobalt bloom" and "nickel bloom"