This began as a comment and grew enough to become a potential "answer."
Here's a bit of physiological trivia that might throw some light (!) on why ~5000K was chosen as a good "correllated colour temperature" standard for critical colour comparison. At that wavelength, human eye receptors in the retina are optimal for red, green, and blue radiation.
Viewing light intensity is the second critical factor when dealing with colour. The light must be bright enough for your visual apparatus to function optimally too. The goal is even diffuse illumination at 500 lux on the surface of a print image to be examined/compared. Not many are aware of this necessity and it often goes ignored. When this is set properly, fix the settings so they won't change. Eliminate any variable natural source of illumination. The goal is consistency and stability. When using fluorescent sources, strive for a CRI (colour rendering index) as high as possible with a minimum of 90%.
When you are working with colour on a monitor, the ambient light colour temperature and intensity can be a factor due also to an "adjacency effect" in non-neutral coloured environments.
TIP: Sitting in front of your monitor OFF in your workspace with the normal working lights ON, what do you see? If you see reflections of yourself and what's behind you, remember that this reflection is additive—and is added to your image when the monitor is on. This will affect your perception of your monitor's output NO MATTER HOW CAREFULLY YOU CALIBRATE IT. Adjust your room lighting so that reflections do not interfere with your onscreen image. Many who work with colour wear black to minimize unwanted colour contamination during evaluation.
There are several fixed "standard" points of colour temperature along the "white" Planc curve on the ICC Colour chart. D65 (daylight) is one. D50 was chosen as another one for reasons mentioned above.
Aside: Printing press operators are accustomed to having a higher ambient colour temperature for more easily seeing low contrast (yellow ink on white) subjects for image alignment. The actual colour management is done electronically in a reserved area of the plant set aside for colour evaluation.
Now, you are ready to turn on your monitor to set its brightness level. While it is warming up a minimum of 30 minutes to stabilize, create a file using something like Photoshop that is 100% Red, 100% Green, and 100% Blue. Display this "document" on your screen while viewing a blank white paper beside your monitor. Shield the monitor from the paper illumination and the paper illumination from the monitor. Adjust the monitor (not the file) to match the subject brightness at 500 lux.