There is no one fixed answer. It's more important to have a full spectrum mid-range light source with a very high CRI. But the industry standard
when comparing on sample to another is 5000K. But a 5000K bulb does not produce a 5000K viewing light because the diffuser inevitably has
a slightly warm cast to it. Color pros will have different light sources available, since they want to match the display or reproduction conditions
of a specific client's needs. I use a variety of light sources in the darkroom so I can evaluate prints analogously. But my personal standard is
4200K because it is a nice compromise between daylight and tungsten illumination. Might lightbox for critical colorwork is very close to true
5000K, with are CRI around 98. Macbeath markets color booths, lightboxes, and targets for industry standard work, which might be informative to study. But casual viewing does not need this kind of accuracy. Critical viewing for precise color reproduction might.