Good question, I don't know. Sounds like it could work. A quick search only gave me some single wavelength yellow ones though. My guess is that they are just more expensive. You'll need two lasers, and also optics compensated for the chromatic shift and suitably coated. That's straightforward, but probably just makes it too expensive for normal laser pointers. But yeah, i don't see a fundamental reason not to make it like this

The vocal sound is much longer in bought than in bot. At least the way I speak and hear it usually Edit: also the t is harder in bot

This, but I think there's even more to it: When checking a plot of wavelengths vs color, (eg https://www.gentec-eo.com/media/3583/commercial_laser_lines.png ) one can see that the region of wavelengths perceived as yellow or cyan is rather narrow compared to red, green, or blue. So if you want a yellow laser, you need a laser running pretty exactly at the desired wavelength, so when frequency doubling around 1140-1180 nm. It is doable, but red or green is just much easier as its a wider range (and suitable lasers have been lon established, e.g. Nd:YAG at 1064nm to be frequency doubled to 532nm, which is a great green).

And I think another point comes into play: yellow is just really not convenient as a laser pointer, when most slides have a bright background. Red and especially green can just be seen mich easier also with low power.

So I think in the end it's a combination of harder/more expensive to engineer and just little demand for it.

I think you're confusing anti matter with dark matter.

But for dark matter you're pretty much right. For example, we can see things orbiting with some speed and can deduce the mass needed for such an orbit. The mass of stuff we see is mich smaller than the needed mass, so apparently there is some stuff that is "dark", i.e. doesn't shine or reflect light, also doesn't absorb. More like glass in this respect, or air. The light just doesn't care about it being present. So physicists started calling it "dark matter" as it doesn't shine, plus it fits well with the fact that we don't know yet what it is.

It would violate charge conservation. The proton (uud) has electric charge +1, while the neutron has electric charge 0. The photon also has charge zero. So if you imagine the process of

p + gamma -> n

and count the electric charges on both sides, they have to be equal. In this case they are not, thus this process is not allowed. Conservation of electric charge is one of the conservation laws that are never broken.

For all such kinds of processes, it's always a good start to check if the charges are consistent (and you might extend this to weak and color charges)