An ongoing unsolved challenge is white balance (Nikon D800E). The camera in auto WB fails way too often, time consuming - Argh. Ignoring PP, in principle what if I placed some calibrated color card into my composition then have the camera use this to calibrate auto WB. To address the software challenges, perhaps I could point my smartphone at this putative color card and have a phone app tell the camera correct settings. Comments ? Already done ? If helpful I'd like to whip up such a card+app
You can't apply a permanent, one-time correction, because light changes. Auto-white balance tries to guess at the color temperature of the light (See Does Auto White-Balance Really Work? How?, as @dpollitt noted), but there's no per-camera correction to auto-wb that would always make sense. If it were as easy as that, Nikon (and all other camera makers) would just have that calibration applied in the first place.
But to make the right adjustment for the light in a given scene, you don't need a colored card, and you don't need an app. You just need a neutral gray card. Your camera, like every DSLR has a built-in way to set a custom white balance from a neutral gray object in a scene.
In your camera, it's called "Preset Manual" (
PRE on the shooting menu.) This is described on page 154 in your camera's manual. You can either read from the scene in front of you, or from a previously-taken photo.
Grey is sufficient because white balance is primarily a shift along an orange-blue axis, with maybe a little bit along magenta-green. Knowing what the center is supposed to be is all the information needed.
For precise color calibration, one would use a more complicated chart. But you don't need that for white balance; you just need the gray card. And conversely, it won't help at all if you photograph the target in one light but then your subject in a different one.