Imagined, written and typed in just on an hour. Strangely, it's not hard to view the world this way.
Disclaimer: Luna belongs to JKR. 'What sound would yellow make' is taken from Terry Pratchett, who always inspires new perspectives.
Luna saw colour everywhere.
She knew that the rest of the world saw it too, but to her it was brighter and clearer and infused everything. Reds and blues and greens and yellows, light and dark and bright and dull; in every mote of light, every pixel, every atom of her existence. Compared with that, others seemed to view their surroundings through dusty eyes. They only saw the colour that was right in front of them, so obvious even the most limited vision couldn't miss it.
She saw the colour in everything.
It was there in magic, a kaleidoscope of vivid shades, shifting with every spell and every wizard. It trailed through the world, shimmering and glistening in everything, opalescent and brilliant. She'd always seen it and when she'd discovered that this living wonder had a name, she'd felt no surprise. What else could it be but magic?
And words. Words were a different kind of magic. Like music. There were so many colours in sounds, in letters, even in lines upon a page. They danced, coalesced in different ways, always composing something new. Every time someone spoke she could cock her head and see the undulating colours their words created, see them twist and swirl in ribbons of paint. If she stood outside and listened to the world, she could see it in her mind's eye, the sharp bold pinpricks of pigment, the softer gentle hues that sometimes faded and sometimes grew into wild, sweeping strokes and far deeper shades.
She'd once read a story where the author had suggested the futility of wondering what sound yellow makes and she'd agreed. It wasn't that she didn't like impossible questions, it was just that it wasn't the right one to ask. He should have wondered what tint birdsong is or which colours the west wind carries with it.
Even people. To be honest, Luna had never understood how others could miss the colour in people. They were filled with it. Everything they did, everything they were was tinged with colour. It changed when they did and it grew brighter and became dull and it was never just one shade, never that simple. They were all made of a myriad of hues, ever shifting, ever growing. People were beautiful and colourful and, oh, there was just so much to see when she looked at them.
She discovered, in time, at Hogwarts, when she finally found friends who didn't seem to mind that they couldn't see the things she did or that she could see what they wouldn't, that the colours were never as bright as they were in people she loved. They were stronger, more defined and so very vibrant, burning and shining in the depths of their beings. She could stare at them, these wonderful, fantastic people, and be mesmerised by the way they made her world blaze with colour. They were bright and soft and strong and bold and light and wild and stark and muted and, oh, they were colourful.
And when they had gone, every single one of them, when they had gone and left her behind, she finally realised what the world looked like to everyone else. Dull half colours, dead colours, a world where everything was tinged with dreariness, cheerless and drab. There was no colour in her magic anymore, no vibrant shimmer of paint in her spells. The energy was there, but the magic was gone. Words were just words, sounds dredged up nothing but the palest, faintest impressions and people were suddenly just walking, talking shadows, living their lives through dusty eyes.
Without them, life simply became reality and all she could see was shades of grey.