I know this is basically heresy to the Spock fandom. I know a lot of people will disagree, and fics will continue to do things exactly the way they always have. But I must speak my truth.
Spock's blood is green but his skin is best described as sallow. Pale with a yellow undertone.
Likewise humans are not honestly all that pink (no matter what Shran says). But we are more pink than Spock is green. We have a pink undertone, but Spock's undertone is yellow.
I've thought it over: the colors of human blood, with and without oxygen; the colors of copper, oxidized and not; the color of the copper-based blood of horseshoe crabs; the optical qualities of human skin. And I offer an explanation.
If you have a lightish skin tone and you flip your forearm over, you'll see blue veins. Which is why you probably grew up thinking unoxygenated blood is blue. It's actually not; it's purple.
What we're seeing is a scattering effect. You know how the sun shines in the atmosphere, and most of the color comes straight through just fine, but the blue covers the whole sky instead of coming straight down with the rest of the sunlight? That's because our atmosphere lets the other colors straight through (the warm white of the sun as seen from Earth) but scatters blue, making it seem like it's coming from everywhere.
Human skin does the same thing to red. While blue comes straight through, as if the skin were transparent, showing clear-edged veins, red is scattered. You won't see your arteries. Instead you see a pink cast that seems to be coming from everywhere.
Importantly, which colors show through and which are scattered has nothing to do with our blood, and everything to do with the optical properties of our skin.
Back to Spock. Oxidized, his blood is grass green. Which is kind of odd when you think about it. Horseshoe crabs have copper-based blood, and it's blue. When it doesn't have oxygen in it, it's pretty much colorless.
And this is the color of oxidized copper. I wouldn't call it grass green. The proper word is verdigris.
So for Spock's blood to be grass green, there's probably something yellow in it. The plasma, or the white blood cells, or whatever.
Unoxygenated, copper is ... well, copper-colored. Orangey-brown. I'm not sure if it's possible for anyone's blood to ever get fully unoxygenated—cells just aren't that efficient. But if we assume Spock's blood is less green and more orange when unoxygenated, we might expect a yellowish-brown, yellow being the only color in both green and copper.
So we just have to assume Spock's skin has optical qualities which allow yellow through more than green or brown. The yellow is scattered, while visible blood vessels (if Spock has any) might be green or brown.
Yes, I'm arguing that Spock blushes yellowish. His ordinary skin tone would darken. You wouldn't have a whole new color showing up.
None of this implies that Spock's mucus membranes (tongue, gums, internal parts of genitals such as a sheathed penis) wouldn't be green. Without the thick, protective Vulcan skin, a lot more would show through.
I'm just saying, Spock looks pale-to-yellow on the show and I'm okay with that. I think science can justify it. (Alternatively, as SPOCKNALIA argues, Vulcan skin is too thick to show much through it, and the yellow tone is Vulcan melanin.)
However, I may still continue to have Spock blush green just for art's sake, and you can too. The only law of fanfic is that your canon is whatever you say it is.
