Are Vulcans always dark haired? Was there any episode or book/story in which some Vulcan was naturally blonde? I mean, if we take in account only the official not the fan-fiction stories.
There was also a Kolinahr Master on Vulcan in The Motion Picture, but I can't quite tell if she's blonde or just has greyed hair:
Occasionally, under some light, T'Pol looks a bit like she's a "dirty blonde", but she has brunette hair. So other than these two possibilities, I know of no others.
Though shown with irregularity, physical diversity among Vulcans is to be expected in the same fashion as it is among Humans. No regularly televised series episode has featured a blonde Vulcan but sandy brown hair has been seen on more than one occasion.
T'Pau, a female Vulcan and Syrrannite (2154)
- While we may not see many blonde (or other hair colors) in Vulcans regularly, it is certain that the genome for such recessive traits exists given the Vulcan genetic history.
Most Vulcans had straight, glossy dark brown or, more commonly, black hair and pale skin with a very subtle greenish tinge, much as the skin of Humans of European descent had a very subtle reddish or pinkish tinge. However, some Vulcans, including Tuvok, had brown skin, tightly coiled black hair, and physiognomic features similar to those found in Humans of African descent.
- Since the Human and Vulcan genomes share many physical traits (sufficient for reproduction with technological support) the capacity for different hair color likely exists.
Memory Alpha provides possible genetic links between Humans, Vulcans and other bipedal species in the Milky Way Galaxy.
Commander Spock once theorized that Sargon's people may have colonized Vulcan some half a million years ago. Sargon believed that Humans and Vulcans might even be descendants of their early travelers. (TOS: "Return to Tomorrow")
With the discovery of ancient humanoid progenitors in the 24th century, most humanoid lifeforms in the known galaxy were found to have a "seed" genetic code guiding their evolution to the humanoid form. (TNG: "The Chase")
- Given the genetic similarities between Vulcans and Humans (close enough to be able to mate with genetic modification), and given that we have seen Vulcans across the entire pigment spectrum and with understanding that Vulcans have had a long species history, as long or longer than modern Humans, it makes sense for them to come in a variety of colors, pigments, and eumelanin densities. Eumelanin is the chemical compound responsible for hair color in Humans.
Human hair color is the pigmentation of hair follicles due to two types of melanin: eumelanin and pheomelanin. Generally, if more eumelanin is present, the color of the hair is darker; if less eumelanin is present, the hair is lighter.
Levels of melanin can vary over time causing a person's hair color to change, and it is possible to have hair follicles of more than one color.
Particular hair colors are associated with ethnic groups. The shades of human hair color are assessed using the Fischer–Saller scale. The Fischer–Saller scale, named after Eugen Fischer and Karl Saller, is used in physical anthropology and medicine to determine the shades of hair color. The scale uses the following designations: A (light blond), B to E (blond), F to L (blond), M to O (dark blond), P to T (brown), U to Y (dark brown/black) and Roman numerals I to IV (red) and V to VI (red blond). Wikipedia: Human Hair Color
Seemingly, by analogy to Humans, the hot climate on Vulcan has left Vulcans without light skin or hair. Any evidence otherwise would be anecdotal at best, dying hair or altering DNA is not unlikely. It never came up in the shows because it's simply not that relevant.
If you're asking this for dressing up: go with anything you like. You're a cool Vulcan.
Sela, the half human half Romulan daughter of alternate universe time travelling Tasha Yar, is blonde like her mother.
Of course Sela could dye her hair, but she said that all that was human in her died when she was four years old and Tasha was executed. If Sela rejected her human heritage and her mother that much, she wouldn't want to dye her hair to look more like Tasha and less like a typical Romulan.
In human genetics, blonde hair is a mostly recessive trait that won't be expressed unless the person inherits the blonde gene from both of the parents. And one would expect that Romulan genetics works almost identically with that of humans - startlingly identical considering they evolved on different planets. One might almost believe that super advanced ancient aliens caused life on the two home worlds to have very similar genetic codes in order for interspecies procreation to be possible.
So it seems highly probably that Sela inherited two sets of blonde hair genes, one from her father and one from her mother. Thus it seems very probable that some Romulans have blonde hair genes and that some Romulans inherit two blonde hair genes and thus have blonde hair.
The Romulans are descended from Vulcans who separated from Vulcan society thousands of years ago at least. Thus it is probable that Blonde hair genes exist among Vulcans and that some Vulcans have blonde hair.