why did the red and green channels separate from the blue in the first set of histograms posted by Guillermo? Has this got something to do with turning off the WB?
It's simply because the B channel is the one that collected less photons in my home's wall. In those histograms you see what the sensor saw, and the B photosites received less photons. After white balance the 3 channels would get perfectly aligned but I deliberately didn't apply WB.
This is a good example to explain why the B channel is said to be noisier than the other two. Actually it is not, what happens in most real life situations (like in my home's wall, under tungsten lighting) is that the B channel records much lower exposure than G and R, making its SNR worse (because given an ISO, SNR only depends on RAW exposure). But at the same level of exposure, all three channels have symmetrical SNR.
A way to obtain UniWB
(WB cancellation) for a digital camera, is to find which precise colour produces the same RAW exposure on all three channels. It should be a colour similar to this:
Shooting at that colour without applying WB on my Canon 350D, I obtained this RAW histogram (nearly perfectly aligned channels):