Brown: it's that dirty, filthy colour, the colour of waste, of a Mudblood, of someone who's been raised with perfectly mundane people who knew nothing whatsoever of magic.
But it's also the colour of shared Chocolate Frogs, deftly passed in the halls when no one can tell because she's a Black and he's, well, not. It's the color of their first meeting, when her sister stuck out her foot in Hogsmeade and plop! There he was in the street, robes all soiled, yet she had lingered with some miserable excuse about tying her trainer (that's how he knew she wasn't like the others) and helped him up without a single word. It's the shade of indescribable comfort, when his Pops owled about Mum not making it, and she was already there with open arms and inexplicably, a Muggle donut, his favorite sweet.
It's also her stockings, he discovers after their excursion by the lake, because the grimy water soaked her robes and she's suddenly struck by something and can't recall the cleansing spell. It's the way he can rest his head just on top of hers, and how their words are soft and gentle, and he feels like he always does when they're so close, he really doesn't deserve someone as powerful, as beautiful, as striking as her. But as his eyes attempt to discern her features in the approaching dusk, he realizes that brown is natural, like the earth beneath his feet, and the messages they shared inside the Frog wrappers, and all those times they mucked up their finals because they spent their nights in the Astronomy Tower instead.
They whisper their spell together, and she's so mesmerized by the flickers of aquamarine that he's positively breathless, and it isn't until she pokes him in his rather rotund belly that he realizes it's time to return to his Hufflepuff home and her Slytherin chambers. He follows her slender form in the darkness, choosing not to tell her about the one remaining star of their spell caught in her hair.