Recently I see more and more (expensive) devices popping up to correct white balance. I see people willing to fork out $50 - $100+ for such devices. I'm simply amazed.
I don't understand, but
1) why are grey cards needed to correct whitebalance?
When i've time, I simply take a photo of a piece of blank white A4 paper, and use it to set custom white balance. It's non reflective, it's portable, and it's very accurate.
When I'm desperate, I simply take a photo of my black bag, and use it to set white balance.
That way, colour purity is maintained. To make it warmer I shoot a piece of very light blue piece of paper, to make it cooler I shoot a slight yellowish piece of paper.
2) Is using a blank A4 paper any less accurate than those expensive devices?
You under expose white and you get a shade of grey.
You overexpose a relative reflective black surface and you get another shade of grey.
From what I understand the idea behind custom WB is simply to balance the different channels to get back equal amounts of Red and Green and Blue. That way, the colour cast is removed (grey is simply a colour where channel R = channel G = channel B)
3) Is there anything wrong with what I'm doing?
4) Is there anything wrong with my understanding of the theory of balancing WB?