As I mentioned there are things that point to biological.. Prince having a white spot is the major piece that I consider, we (to my knowledge) are not 100% sure that it is vitiligo though right? There is a handful of diseases that cause white patches that appear similar as Vitiligo (Versicolor/Nevus depigmentosus/Hypopigmentation etc.).. Whatever the case is there's many variables to consider..
Three family members (Paris, Jermaine and La Toya) have confirmed that it is vitiligo. Dermatologists who have seen the pictures of it said they believe it's vitiligo.
I have looked up the diseases you mentioned on Wikipedia. The symptoms of versicolor:
The symptoms of this condition include:
Occasional fine scaling of the skin producing a very superficial ash-like scale
Pale, dark tan, or pink in color, with a reddish undertone that can darken when the patient is overheated, such as in a hot shower or during/after exercise, tanning typically makes the affected areas contrast more starkly with the surrounding skin.[9]
Sharp border [10]
Pityriasis versicolor is more common in hot, humid climates or in those who sweat heavily, so it may recur each summer.[10]
The yeasts can often be seen under the microscope within the lesions and typically have a so-called "spaghetti and meatball appearance"[11] as the round yeasts produce filaments.
In people with dark skin tones, pigmentary changes such as hypopigmentation (loss of color) are common, while in those with lighter skin color, hyperpigmentation (increase in skin color) is more common. These discolorations have led to the term "sun fungus".
This disease can result in a loss of color for dark skin, but for someone with light skin like Prince, it usually does the contrary and darkens the affected skin. It's a fungus and it's treatable, so Prince wouldn't have had those spots for so many years. Also, it makes lighter spots on dark skin, but it doesn't seem to make skin
white the way vitiligo does. Prince's spots are not just lighter than his skin tone, they are really white, as white as can be. Also, his skin is perfectly smooth on the affected areas, while this fungal disease makes skin a bit scaly.
The symptoms of nevus depigmentosus:
Nevus depigmentosus or nevus achromicus[1] is a depigmentation problem in skin which can be easily differentiated from vitiligo. Although age factor has not much involvement in the nevus depigmentosus but in about 19% of the cases these are noted at birth. Their size may however grow in proportion to growth of the body. The distribution is also fairly stable and are nonprogressive hypopigmented patches.
These are localized white spots on skin which may affect any area of the body, but these white spots are quite stable lesions.[2] In the majority of patients, the lesions are not completely achromic, but are hypopigmented and resemble splashed paint.[6] The individual lesions are permanent and there are no effective therapies for re-pigmenting this nevus.[6] If there is hair in an affected area, it is usually colourless or white.
This one would be the closest one to vitiligo. The main difference would be in the stability. If it's nevus depigmentosus, the spots don't change or grow. Dermatologists can confirm whether it's nevus depigmentosus or vitiligo by looking at it with a Wood's lamp.
I will say however that the locations of Prince's spots are very typical of the first stages of vitiligo (hands, armpits, neck,...) Also, his spots seem to have appeared progressively over the years, unlike nevus depigmentosus. He didn't have those white spots under his arms, on his hands or on his neck as an infant.
As for hypopigmentation, Wikipedia says that it is not a disease, but a symptom. A symptom of vitiligo, albinism and tinea versicolor among others.