Just one does lighten the front; I mean, it's a matter of taste, I'm sure many riders like the solid look of the 16-inch front with black walls, but I think it looks kind of dumpy, and the whitewalls really spruce things up.
I'd probably get a rear whitewall as well though (easy choice for now since I have to replace both anyway). Assuming I can get some of those Avon Radials... they're thin on the ground, even the Belfast guy says he'll have to check, plus they can't really arrange shipping and the like as they're not set up for online ordering. I've been badgering my local guy too to get some, and an online store in the UK...