Movie settings, including Set Custom White Balance, should be done in P, A, S, or M mode.
Turn the Mode Dial to Movie to choose whether to use P, A, S, or M settings for video.
With the Mode Dial left at the Movie position, Set CWB is grayed out, however, 3 CWB done in P, A, S, or M will still be selectable.
From my observation, the Sony A7S with its amazing low-light capabilities, FF IQ and DOF is becoming the preferred mirrorless 4K camera amongst professional filmmakers and the saving on lighting (purchasing, renting, transporting) can easily offset the cost of the external recorder. For now GH4 with internal 4K recording will be the preferred 4K camera amongst divers until Sony delivers internal 4K recording for A7S Mark 2 (??).
Since switching system is expensive, looking long term, the Sony mirrorless system has the most potential and more future proof, IMO.
I heard on one review that manual white balance was not possible while doing video with the A7S - if that's true I think it would be a deal breaker..
My hand-on experience: In the Movie mode, A7S can perform all of the White Balance functions except Set CWB. The easy workaround is to set up to three CWB settings in P, S, A, or M and all three CWB settings will be accessible in the Movie mode. Hopefully, this little inconvenience will be corrected in the next firmware update. BTW, A7S also has an Auto Underwater WB function, but I don't know how well it works.
Did a side-by-side comparison between the Zeiss 12mm and the Sony 10-18mm above water. Not surprisingly the Zeiss 12mm F2.8 being a prime has sharper corners than the 10-18mm @ 12mm F4. For pixel peepers who don't mind noisy AF, the Zeiss 12mm is a great choice. Since the Sony 10-18mm has silent autofocus, smoother video AF performance, noticeably wider FOV, and Optical SteadyShot, it's a more useful lens overall and definitely a better video lens IMO.
The Zeiss 32mm F1.8 is also very good optically just like the 12mm, and its AF is also noisy just like the 12mm - perfect for photography or shooting video without AF. For video overall, the Sony E 35mm F1.8 OSS is a better choise IMO.
I think the combination of cyan filter for the light and CWB seems like a very logical solution to balance the light-lit foreground and the sun-lit background colors.
Stills and videos taken with AWB always turn out unnaturally blue so I prefer taking my time to set and fine tune Custom White Balance to ensure the colors come out as close to what I see as possible. Beside, I won't be able to remember the precise colors hours, days, weeks, or months later. By the way, I cannot adjust the colors in the above AWB image anywhere close to the CWB, using either Photoshop or Final Cut Pro X. Isn't setting CWB in camera = adjusting colors in RAW before most of the color information is thrown away during the JPEG and video-compression process!
Large sensors' better high-iso performance can make a huge difference, diving early morning and late afternoon when CWB is pushed to the limits. Just saying...