Iliah Borg writes:
Knowing that green is more sensitive, one of the tricks to equalize channels better while shooting with daylight/flash is to use a magenta filter, about 30 to 40 percent depending on the camera (CC30m, CC40m; P instead of m if the filters are from Germany). This will also improve colour reproduction if the filter is something like Lee are making. I use Lee gels.
I did some experimentation. The first camera I tried, the Sony NEX-7, turned out to be a bad camera for this technique. The reason the NEX-7 isn’t well-matched to this approach is that it has a live histogram in the finder, which is great for ETTR, but it’s a luminance-only histogram. Green is a large part of luminance, and, if you’re pretty sure the green channel will be the first to clip, which is the case most of the time with no filter, you can safely move the luminance histogram to the right until just before the rightmost bucket starts to fill.
I put a Tiffen CC30M on the Zeiss 24, and made some test shots that included blue sky using the finder histogram for exposure. When I reviewed the images in the camera, I saw that the blue channel was badly clipped.
That said, if you’re setting the exposure by making a trial shot, looking at the color histogram, and adjusting, then the technique that Iliah recommends works well once you’ve calibrated your in-camera histogram for UniWB.