Connecting it directly in a VGA output, it will bridge and convert the RGB signals to a passive equivalent luminance value with a few resistors, then you just have to select the monochrome output by combining the primary colors red, green and blue using the some buttons.
The luminance is calculated passively and approximately to the following formula:
0.2126*R + 0.7152*G + 0.0722*B
The schottky diodes are intended to prevent the RGB signal from going back and mixing when the switch is selected in color:

In a new improved and updated revision of the board, the double passive components shown in the diagram serve to have footprints with a mix of encapsulation technologies (SMD and TH), in order to make the assembly task more flexible.
All changes from first revision:
- The RGB resistors provided are 90% smaller in value. The filtered output signal is brighter, it can be considered an improvement.
- The VGA input connector is now also female, to prioritize use in a case and more accessible to the user. However, a Mini Gender Changer is also used but this time it must be male-male type, to connect directly to the video output or to the monitor input, and can be left permanently.
- New PCB design with through hole technology and mixed SMD, for an assembly task adapted to the needs of each one.
In the following video can be seen in operation with the first revision. The color mode display is affected by poor lighting, where saturated colors are displayed. With this lighting I wanted to highlight the monochrome mode:
Now with 3D case too, thanks to Agustin Gallego for designing it:
I'm struggling (to put it quite diplomatically!) to understand your schematic as posted... it unfortunately follows the current horrific convention of "this is a diagram of visually disconnected parts and signal labels and it won't make any sense until you stick it into a computer program, unless you spend multiple hours working out by label exactly which signal goes where and then redraw it by hand".
I learned to draw schematics based on the way they did it for 80s home microcomputers. Go look at C64 schematics online, you'll see what I mean fairly quickly. I can read those... and the ones for my Amstrad PPC640... and for the KIM-1 and Acorn System 1 and dang near everything else from that era...
Yet when I first saw the schematics, as provided on the UK Lo-Tech site, for their version of the famous XTIDE card, it took all of maybe three seconds for my eyes to cross... I spent literally three or four afternoons redrawing it to be human-readable. It was entirely inscrutable otherwise, except to whatever abject coding failure of an electronic design suite spat it out originally.
In the case of your schematic, I see two VGA connectors, four resistors, four Schottky diodes, a 4-position DIP switch, and a slice of mystery meatloaf in the lower-right corner that apparently escaped the local elementary school cafeteria and struck out on its own. I have no idea what on Earth (or elsewhere, for that matter!) that box is or what it does, as it's not even labeled, and I cannot fathom for the life of me how any of it connects to the rest.
Forgive the soapbox, but... this isn't a matter of me being "old-fashioned" and needing to "get with the times". This is not an "okay boomer" sort of thing (and I'm a millennial anyways). This is a simple basic fact of design, that is pretty well universal across all fields under that overarching term -- or, rather, an absolutely textbook example of what happens when you ignore that rule.
The rule is very simple: if you want to communicate an idea, you have to do so in a way that your intended audience can understand.
Schematic diagrams are what amounts to a visual descriptive language. What I'm seeing here is a bit like trying to speak English but without using any verbs... could you read (in a way that let you understand it, obviously!) Shakespeare like that? or Tolkien? or Poe, or Arthur C Clarke, or Jules Verne, or Agatha Christie, or anyone else for that matter...? I couldn't, any more than I can understand this.
You wouldn't use an operating system with an incomprehensible interface (and neither would I!)... so why make schematics like that?
Please redraw your schematic in human-readable form. Alternatively, if you haven't the time, provide me with a complete text description and I will draw it for you (I'm an artist as much as I am an electronics and computers nerd, so straight lines and grid paper are very much the order of the day!) -- but you must also promise to upload the schematic I provide, for others to use, once I've done it. I believe quite deeply in the sharing and spreafing of knowledge, not hiding or restricting it for any sort of private or personal gain.