childrule

This implies that the pencils evolved the same color receptors and optical neurons that humans did

Which by far is the least concerning part of sentient pencils with arms and legs spawning offsprings in the white void?
- Shhh I'm almost there
Hey, we can't prove they didn't
To see a difference in saturation as less significant than a difference in hue? Yeah I guess but that doesn't sound that unlikely for something to evolve with, especially as perceived saturation can easily be changed by a glare over the surface or something.
From a spectral perspective, there's one that absorbs a lot of red and a bit of green (the blue one), and one that absorbs a lot of blue (the yellow one). Mixing them together, as long as particles overlap eachother and light has to travel through multiple, should result in a greenish hue. The center of the spectrum will still be in a different place from either of the parents even with more than 3 color receptors, while in the top example the center of the spectrum stays in the same space and only the 'spread' on the spectrum changes, even with more than 3 color receptors.

Why are the younger pencils pared-down? Do they age backwards?

oh yeah why aren't baby pencils the larger ones
- The entire comics joke is that these pencils somehow wouldn't understand that blue + yellow = green despite presumably existing in a society where this happens regularly so I don't think its that reasonable to try and treat the comic that seriously.
Maybe they should be tiny but unsharpened.

Fixed.

My sister looks white/east Asian, my brother in law, german with maybe a Korean twist. Thier kid? 1000% Korean with red hair and green eyes. Genetics

My wife is 1/8 native American, but has red hair and green eyes. She is able to tan instead of just burning, which is nice.

What is the punch line?

The joke comes from how colors are mixed. Red and white will always make pink but green and blue make different colors depending on if you're adding or subtracting colors.
Additive color (RGB) is when you shine red green and blue flashlights at a wall. If all three meet, you get white light.
Subtractive color (CMY) is where you add pigments to a surface that absorb certain colors. Adding cyan, magenta, and yellow pigments results in a black surface since all light gets absorbed. (Normally it's CMYK for printing where K is black so you can get deeper blacks)
- But if you combine blue pencil and yellow pencil marks you get green (on white paper). You're not working with light beams here. Maybe the blue pencil is colorblind?
Maybe... that green doesn't look like yellow or blue but is still what you get when you mix them? IDK, honestly 🤷
- In the most technical way, that green is what you would get.
  If you blend the colors with a gradient, you get these, which to me feels like a more natural blend:
  More muddied and dull versus the technically correct color.
Found an explanation from Medium:
The blue pencil father suspects infidelity seeing the green baby, but green is actually the correct result of blue + yellow. He’s wrong about cheating because he doesn’t understand basic color theory.
- Yeah that’s the joke
  Green doesn’t look like blue or yellow
- I figured he was red green colorblind.
Yeah, I'm not sure anyone here actually knows, even that Medium article. It honestly would make perfect sense and ultimately be a mediocrely funny comic if the baby was orange and you could see the red guy in the background of the last panel. As is, without any additional dialogue or captioning, it's too vague and open to interpretation.
- Yes, that's what I thought as well. At worst, I did not understand color theory as good as I thought. At best, the comic is just not that funny.
you mix yellow and blue to get green (at least elementary school usually teaches it this way)
- And you mix red and white to make pink. That is not a punch line.

The punnet square checks out

Now do Red and Blue.