Green
It's a rather symbolic colour, isn't it? Green. Health, nature, comfort, and so on, so forth. All those nice, happy things, that make you want to skip through meadows and sing songs with baby blue birds, and dance with bunny rabbits with far too large a pair of ears.
Lily Luna was not setting a good example for the colour green. In fact, one could say she was all the bad things people associated with green: evil, jealous, an object of temptation, Slytherin, in the very least. Which was partially correct, she was, as of September 1st three years ago, Slytherin. And yes, one could say she was quite the lustful little fox (or, as the rest of the school would have it, snake). And yes, one could definitely say she was evil. Well, mischievously determined, but that was a very blurry line to the youngest Potter, or really anyone looking at it. Really, no-one's ever been sure that line's ever been completely clear to the naked eye, but that's another story for another time. But if there is one thing, in the whole wide world, Lily Luna Potter will never be, or achieve to be, it is jealous.
No, little Lily's smarter than that. After all, to be jealous, you need to want something passionately first, and Lily didn't want anything, or anyone. Of course not, she was the wanted. And those who are wanted never want anything in return, in knowing that their popularity is still higher than any height her brother could fly. After all, the wanted have everything. That's what makes them wanted. One wouldn't want to be the small, snivelling, scarred child with the brains that worked the same speed as a snail. One always wants to be the beautiful, red-headed, face-of-an-angle-girl with the large bust. They wanted to be Lily. They were jealous of Lily.
Besides, even if she did want someone (or something), there's still a second step to jealousy: to allow oneself to take their defences down, and let that person (or object! It could be an object) open their chest, open their heart, and mess it all up.
And Lily never took down her defences, never allowed herself to be jealous, because that only lead to one thing and one thing only: defeat.
And Lily could not be defeated. She would not allow it.
So, why was it, that is, if Lily was smart enough to know this, beautiful enough to be wanted, and strategic enough to keep her defences up, why did her vain little heart feel a tug when she caught the Malfoy boy's eye locked onto her disgusting cousin? If this was all true, what was this strange pang when she saw the Malfoy's hands tangled in that brunette bitch's hair as they talked through tongue? How, then, Little Miss Potter, could you feel so goddamn delighted, and hopeful, when you watch that infamous pair, who occupy your mind so much, bicker and squabble in what seems like pure hate, only to deflate yourself with a disappointment-shaped pin as they forgive each over in their arms.
Really, Lily, if you were so smart, and so beautiful, and so bloody perfect, you wouldn't even have those feelings.