The appearance of Romulans has been limited by their being portrayed by human actors (in blatant Federation propaganda, no less), but something I have long been obsessed with is their green blood and how you would go about portraying a person with green blood. A lot of the redness in human skin is due to the redness of human blood, and areas that are more heavily vascularized like mucous membranes (ex: lips, genitals, etc) tend to be a lot redder than the surrounding areas in fairer skinned humans. Some of the red-ish color in human skin can be due to pheomelanin, but this is generally more of an orange color (like the color of freckles) than what you get from the blood visible in the more highly vascularized areas. Darker skinned humans still generally have a reddish undercast to their skin, because of their red blood, but darker skin is less translucent because of the role of melanin and other pigments in protecting the skin from UV so the blood is less visible. But even the darkest humans have red tongues and gums. Of course, then, very fair skinned Romulans would have a lot of green in their cheeks, green lips, green nipples, etc. And even the darkest skinned Romulans with lots of pigmentation with not a lot of visible vascularization would have green tongues and gums. A portrayal like this is what I consider the “ideal” or “true” portrayal of a Romulan, and the standard against which I personally judge all makeup on human actors portraying Romulans.
In TOS, they do put green makeup on the Romulans (and Vulcans); mostly eyeshadow (for some reason), and I think they generally tried to cast more naturally olive-complexioned humans so they wouldn’t require as much makeup to cover any redness in their faces. No redness visible in the cheeks, they seemed careful to avoid that. Curiously, they never opted to cover the redness in the actors’ lips in TOS, so the lips look pretty incongruous. But, given a lot of the limitations in the makeup technology of the time, it does a decent job at portraying the *idea* of a green-blooded people.
In TNG, they definitely got a lot greener in many respects, and again they seemed to preferentially cast more olive-complexioned people so they wouldn’t have as much redness to cover up. But curiously, red lipstick was often applied to the actors, which seems simply absurd to me. If Romulans (and Vulcans) used cosmetics, they would paint their lips green – of course. I like to joke that Romulans and Vulcans went through a period of fashion where everything Human and Earthling was hip and just the coolest – so painting your lips red became all the rage (“Look at me, I’m a human!”) of course I don’t actually believe this. Simply more evidence of the limitations of Romulans being portrayed by humans – why they don’t just hire Romulan actors to play Romulan characters already is beyond me.
Starting with around DS9/Voyager era, makeup technology had improved, but also that was when (I think) the makeup artists really started putting a lot of effort into imagining a truly green-blooded person. They even cast fair skinned Romulans (like Letant and Vreenak who are both very fair; and as a great detail Vreenak even seems to have that green “Romulan Flush” going in the scene where he is drinking kali-fal), but they blotted out all the redness in the actor’s cheeks and lips and around the eyes with a neutral skin tone to wash it out, and then went back over it with green wherever there was red. It is really impressive. All the Romulans and Vulcans now have a generally green undercast to their skintone, and no more red lipstick! Vulcan women in DS9 paint their lips green, and it looks fantastic. It ranges from more brown-green to a more vibrant green, same as with Earthling cosmetics (where lipstick can range from red-brown to very red). It also makes sense because Romulans with lots of orange pheomelanin (lips are an area that can have more pheomelanin) would probably have those areas coming out looking more brown, due to the green blood undercast on the orange surface pigment. And in an interesting attention to detail, Romulan women in DS9 appear not to use cosmetics. This is portrayed consistently enough that I think it was intentional – to highlight some of the cultural differences between Vulcans and Romulans. Vulcans have more rigid and prescribed gender roles, and Romulans have looser gender roles. Vulcan women use lots of cosmetics, Romulan women use none. An interesting way to use theatrical cosmetics and “show, don’t tell” and execute some situational storytelling.
But then, starting with Enterprise onward, we’ve been getting progressively less green Romulans and Vulcans. In Enterprise it is like a callback to the kind-of-green but with red lips, which I understand it takes place before TOS so the aesthetic choice may have been intentional in this regard. Well maybe the Human makeup fad was going on then, I don’t know 😉