Yep, most non original CGA cards do not emulate that unofficial RED Cyan White Black.

And many modern sVGA-s only have black, cyan, magenta, white anyway, so... not even those dark and swampy black, red, yellow, green games do not look normal.

Hi, thanks for the reply! - I agree, most VGA cards don't to anything but the default palette.
If the PC games use the BIOS, the second palette (0) sometimes works on VGA.
My very old VGA card, for example, comes with a mode utility to improve compatibility.
It can put the card in a special CGA mode, so that some old games can change things directly via registers.
In this case, it didn't seem to work, though. At least, there's no visual difference..
Edit: Now that I'm thinking of it, the game perhaps sets mode 5.

https://en.wikipedia.org/wiki/Color_Graphics_Adapter

Yes, Popcorn uses mode 5. On a CGA-compatible video card mode 5 displays red instead of magenta on an RGB monitor. Few VGA cards use the different color for mode 5, although some might with special CGA-compatibility settings. Another significant difference between CGA and VGA is the 60 Hz vs. 70 Hz refresh rate; and Popcorn does in fact do some timing based on the vertical retrace.

BTW, DOSBox SVN has a small change for the CGA machine type to help Popcorn's default speed (alterable by its separate POPSPEED program). So, for the reasons of color, display timing, and default speed, machine=cga with around 300 cycles is recommended for Popcorn.

Part of the reason for the wrong colouring being shown in a lot of screenshots is because one of the original two creators of PopCorn who's been keeping it alive on his site along with Flash and mobile versions has been using the cyan-magenta-white palette, so even the game's CREATOR is getting it wrong! ;D

I see. That's kinda ironic, isn't it ? 😉
Makes me wonder as to why they used mode 5 in the first place.

Oddly enough, The CGA Compatibility Tester somehow manages to display
the third palette on my VGA card (in special CGA emulation mode, again).

@ripsaw8080 Thank you very much for the explanation and the tip! 😀

Update.

After running The CGA Compatibility Tester, performing the tests in Color Select-Register->Medium-res palettes,
the third palette now works in Popcorn (on my VGA card in emulation mode). 😳

I'm not certain which bits it did change in particular, but it helped.
Would be cool if that code for the palette test could be turned into a little utility (cycles palettes, then quits).

Edit: Took a video: https://youtu.be/cu88uKmqlEU