I'm going to go "techie" here, but I researched this a bit. Yes, natural light is best, but often times bathrooms and makeup rooms don't have the capabilities to allow only natural sunlight. As such, here's what I dug up. Hope it helps!
"Too often studio designers will place 25-40-watt incadescent bulbs around a mirror in a makeup area. Not only do they provide insufficient light, but also their color temperature is too low for proper color judging. They also produce excessive radieant heat, thereby warming the area unnecessarily.
A far better system employs special fluorescent bulbs. Now although color temperature is the measure of the energy distributed of the spectral range--that is, color quality of a light source with a continuous spectrum (such as tungsten incandescence)--an apparent color temperature can be assigned to lights like flourescents for comparative values.
Fluorescent bulbs diffuse light with a scattering or criss-crossing of light rays that produces a general illumination rather than a direct radiation. Thus, there is no filament that burns or incandesces under the application of electrical excitation but only the fluorescence of certain gases that produces the light values. Fluorescents cannot have their light directed or focused as can incandescent lighting. Many people have a poor knowledge of the varieties of fluorescent bulbs that are available as most hardware and even electrical supply stores stock and sell only one type of bulb--normally, a cool white type of bulb, which as little red in its spectrum so yellow and green portions predominate indicates the apparent color temperature of available fluorescent bulbs and adds a better judging rating known as the Color Rendering Index (CRI) (which has to do with the way that color materials appear under a specific light source). A comparative listing of natural sunlight shows the optimum rating of a CRI of 100 for comparison.
The VITA-LITE has the closest CRI to natural sunlight and apparent color temperature within its exa t range. As such, f these bulbs are the best type to place in a makeup room to judge the color of makeup and hair as well as to provide a proper amount of light with the least amount of heat. For example, a mirror 4ft high and 3ft wide can be placed between two 4-fot and one 2-foot fixtures. With the makeup chair some 4 feet way, the amount of wattage is only 200 and rates about 114 ft. candles, which is quite sufficient for judging and applying most makeups.
As the color temperature of sunlight is approximated by these lamps, they provide the optimum lighting conditions in color quality and color matching for makeup, hair, or wardrobe. Any makeup done under regular incandescent bulbs (2,900 degrees K) will not provide the proper color temperature to judge red or pink tones, such as with lip and cheek colors, so when the subject is taken out of doors after having makeup applied under incandescent lighting, the color match can be off or the cheek color might appear streaked." --The Technique of the Professional Makeup Artist by Vincent J-R Kehoe