River Song wore black on her wedding day.
The Doctor thinks it is appropriate, considering their back-to-front, upside-down tangled whirlwind of a married life. After all, they never meet in the right order—calling the other to another point in time is like playing roulette: They never know what they're going to get. The same questions plague them every time: Will she still be recovering from her brainwashing? Will he know her identity? Will he/she be married yet?
And, by marrying him, she'd more or less etched her fate out in stone.
The day he married her was the day he truly started to grieve for her.
Sometimes it hurts to breathe, knowing that she'd been ripped from him before he'd even begun. Oh, he knew that she had had an entire life behind her, rich with love and adventure and family (he'd make sure of it)—but he hadn't started yet. "You'll see me again," she'd said. The "But this is the last time I'll see you," had been left unspoken.
He always destroyed them in the end.
(Martha had been lucky to get out while she still could.)
They're sitting on the steps to the TARDIS console, playing Cat's Cradle. After a long day, saving a mine from a chemical explosion on Draavis VII, the Doctor had piloted the TARDIS to the twentieth century and stepped out to get them Chinese food while River showered off the grime from the trip. They'd eaten in record time, having skipped both breakfast and lunch in light of the incident (the Doctor had suggested a breakfast date in France in the twenty-third century, but got the flight wrong). When they'd finished eating, he had extracted the string from his pocket and began playing to keep himself occupied.
Sometimes, the Doctor forgets that River has never had a true childhood. He spied her watching him thoughtfully, as he twisted the string into a complex pattern (one he'd invented when he was a child. No one else had ever been able to learn it) and asked her if she wanted to play. She had blinked at him, before admitting that she didn't know how.
Now, they're laughing as she makes another mistake. She'd given up being frustrated with herself about half an hour ago, and is now enjoying the simplicity and repetitive movements of the game. He likes her best like this, carefree and smiling—oh, she's lovely in any mood, but relaxed and happy is always the way he likes people best.
She's wearing green now, her blue dress from earlier thoroughly ruined. It's not anything fancy—a plain green dress that he thinks Amy bought for her one Christmas in her future. She'd left it on his bedroom floor one night not long after receiving it, after he'd peeled it off of her. (If she had been any other woman, the Doctor thinks, she would have demanded an explanation for a strange article of clothing in his personal closet. But, due to the nature of their timelines, she didn't bother to ask.)
He likes her best in rich colours. Colours mean life. Deep reds and dark blues, soft greens and earthy browns. (His mind helpfully points out that she looks best in nothing at all, but he pushes the thought away. Maybe later.) He doesn't object to the black, either—she looks good in it.
Black isn't white.
He hates her in white.
Luckily, she doesn't have much occasion to wear it—if she does, she offsets it with something dark (always so fashionable, he thinks with a grin). He's rarely seen her in nothing but white. There was that one time, on Sylii II—he may have accidentally-on-purpose bumped her into that mud pit. She'd made him buy her a new dress to replace the ruined one. It had been silver. He'd liked it—she'd worn it dancing the night before he'd ruined the white one in his timestream.
So, yes, he thinks that it's appropriate that she'd worn black at their wedding, and not white. The human symbolism that white equals purity would have been laughable in their case, anyway. If there is one thing that River Song has never been, it is pure—both of them have been twisted and blackened and corrupted in so many ways that he doesn't think there's a pure bone in either of their bodies. He thinks it's why they fit so well.
White doesn't suit her. That's what he tells her when she wears it. But, in reality, he has another, far more potent reason for disliking her in white—one he can't ever tell her. He tries not to think about it, drives it out of his mind with a childish game or a wisecrack, or even an explosion or two, if he's desperate. It's not happened for her yet, and until the time comes for him to take her to the Towers, he's just going to avoid seeing her in white as much as possible.
Because when she wears it, he can't help but think about it.
She'd died in white.