Sometimes when measuring and then taking photographs of extremely non-even spectral power distribution light sources, such as some LEDs and fluorescents, the output image "just doesn't look completely white".
This can happen due to much of a spectral mismatch between the camera sensor and light spectrum.
What does that mean?
Camera has different light sensitivity for different colors (or wavelengths) of light. And e.g. LED light outputs more light at some wavelengths than other. The cheap ones usually have large spike in blue light area.
When one such "low" and "high" are combined (it can happen rarely, with just some lights) you get a color mismatch.
If we list all possible reasons for discrepancy between what color meter measures and what the camera "sees":
- different sensitivity to different wavelength
- different color handling
- different internal WB measuring algorithms.
Even when using multiple units of the same camera model, there could be some very small but noticeable color discrepancy between the final photographs.
The proof is if you take a look at the histogram of the photograph, shot under extremely non-even and low CRI light source (such as cheap LED lights). Fine tuning your RAW file in the Adobe Lightroom, you can match R, G and B channels in the mediums area, but you still have large channel mismatch in the highlights section:
So it is impossible to make a photograph under this particular light and get a neutral white result in dark, medium and highlight areas.
And that's why also the WB measured by the camera itself isn't completely correct.
That doesn't mean that the color temperature meter is useless.
As Adam Wilt says in his comparison test between Lightning Passport and Sekonic C-700 spectrometers and Minolta Color Meter II and Sekonic C-500 tristimulus meters, "The more lumpy your source’s spectrum is, the more a slight difference in the spectral responses of your camera and meter will be likely to cause visible mismatches between what the meter predicts and what the camera sees."
You can read Adam's complete test here: On the Color of Light, and the Measurement Thereof.
The single most important aspect of using a handheld Color Temperature meter is to use it in real life situations, with a camera you are used to, and to have it as a starting point in getting the image suited to your taste and desire.
It will help you become a better photographer or cinematographer.
For further reading, check the article COLOR TEMPERATURE: Real life measurements, accuracy and comparisons.