The first improvement from yellowish to blueish *was* made using an internal camera setting for white balance under halogen lighting (as opposed to auto white balance). The second improvement to a much whiter white was made using software. I'm not sure if it's possible to manually tweak the white balance in the camera, but this is near enough for my purposes until I reach the next level of fussy.

As for the filter, yes, I will have to get the focus, lock it there, and then move the filter in front of the lens. It's a standard trick for taking shots through windows or off-centre subjects. Not something I'd want to do in the field for action shots, but manageable on a tabletop.

Thanks for the advice! My knowledge is *very* spotty and I never know where the next critical nugget of information is going to come from.