You can use gray or white. I don't think there's anything wrong either way. But, I'll share my experience with this insanity.
I prefer to use an 18% gray card for the majority of white balance adjustments/readings. Mostly due to white cards picking up too much reflected light. If the scene has a lot of ambient object color contamination I've found white to be a bit inaccurate on numerous types of materials. I am often angling my gray card towards the key light or grabbing a specific shot at a better angle for color reference.
I tend to slap a white and gray card on the back of my slate and flip it just before a take so I have the option for both when color correcting later.
While I do prefer gray, I'll give you an example where white was very useful recently. I was shooting an interior with one key, one fill, and ambient daylight fill through windows and a screen door. I noticed that the sun bounced off the aged stucco wall and was producing a greenish yellow light. You could see this by rotating the white card towards my balanced K5500 key light. Added a bit of filtration to the key and fill and to get a bit more green in there. When color correcting in post I balanced to the bounced light off the wall. Adding some color to the lights and paying attention to color cast in the reflected daylight solved a potentially hazardous skin tone and color cast issue later on.
Beyond white balance, it is useful to have a white and gray reference for exposure and metering purposes, especially when lighting for close ups and tighter shots. For precision work though I use a full color card. I keep a Passport Color Checker on me pretty much at all times. Handy little bugger. Rugged too.
I'm pretty darn sure this is my most photographed subject.
Nice thorough test.
There was an interesting discussion here on REDuser back in 2011 where I shared a few notes.
Short story, I prefer using 18% Gray exposed properly as my White Balance Reference.
Medium length story, I've found it useful to have both 18% Gray and something White to balance to and/or problem solve tricky situations.
Long story short story, I pretty much always have a Color Chart of some sort on me. In my pocket, in my kit, on the slate, wherever. It's what I use the most.
Here's my old post:
If you guys have been following what I've been doing here on the boards I can recommend the Passport Color Checker (there's a new "Video" one now too), DataColor SpyderCHECKR, and any of the DSC LABs charts like the Toni or ChromaDuMonde 12 + 4 or the Front Box One Shot which is shown in the article.
What Art's locked onto here is the subtle nuance that can be found by matching to a properly exposed Middle Gray target.
In these times where so many are critical of skin tones, besides the light quality, a decent White Balance method and developed Exposure Logic is important if you are looking to maintain consistent and high quality results.
Here's a grab out of RED Weapon. Exposed to meter, confirmed to 18% Gray using the In Camera Exposure Tools, and White Balanced in camera to 18% Gray. This was ISO 800 DRAGONcolor2/REDgamma4 with nothing else done.
My opinion for color-correction is that I'd rather have a 10-step grayscale chart:
