I notice in my latest read that some indie filmmaking crews and video podcast production teams are using something called QP CARDs. Which are available in Grey Scale as well as Colour variations to help people colour balance all of their shots / pictures before further image processing begins in Post.
Richard Harrington mentions in Producing Video Podcasts that some people actually tape them to their clapper boards to mark the beginning and ends of shots and I think this is a great idea! It will certainly make post production easier if I can bath process shots and match them accurately.
I actually use Warm Cards when I want to achieve a more creative look or add a certain off White Balance in a shoot, and can definitely recommend them, but when I want true colour I think QP CARD will be the way to go.