You must mean VGA since CGA was not RGB. 
I’m really no expert but I think what you are seeing is just the scanline. If you remove the scanlines a slot mask should give you what you expect.
That’s the comical thing. In my original manual and the copy found online, Tandy says this is a 16 color RGB monitor:
https://manuals.plus/m/592b4454659e114409ba085914c51a7bd0f03b1158a49d7c84521bf1bd10611f
However, I recall in Radio Shack catalog listings and ads from way back in the day, and in present websites nowadays, they often refer to this as a CGA monitor(likely because they are of the Amiga breed and this term seems to be used loosely in that segment of retro PC society), whereas on the PC scene, CGA refers to 4 colors or composite out to a TV that produced more colors through artifacting:
Basically, the monitor connector has 9 pins which was common among CGA monitors of the time, unlike later VGA which used 15 pins. The Tandy 16 color standard straddled EGA and CGA, and competed originally with the PC Jr. upon release, which also had 16 color capability, which was remarkable way back in 1984 when every other PC could only display 4 colors. The Tandy 16 color mode is incompatible with EGA, and special drivers had to be written to output properly.
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True. Tandy used some tricks to squeeze 16 colors out of a 4 color display. Still… not RGB in any way.
You will not be able to reproduce this with any existing shader, a slot mask is as close as you can get AFAIK.
oh the nintendo vs. redcaps ?
they are part of my MBZ preset/overlay pack
there is a bunch more besides those in the pack , and you could look into my pack to see how params and the wildcard stuff works 
there are three links if you wanna check everything out scroll through and read the first post complete
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Awesome, I’ll definitely have a look at that, they look really cool.
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Hey!
Great to know it’s on the list for a fix, I thought it was a problem on my end.
Thanks again for this fine work, this shader alone has lead me to play more and finish many games I had pending from my childhood 
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Welcome!
Looks like your screen is wider than 16x9. At present there are no off screen elements to fill in the space on wider screens. You can turn on the “Mirrored” parameter for the Device layer… or zoom in using “Veiwport Zoom” until the space is filled.
Eventually I will update the handhelds that are cut off but there is no ETA. Probably later than sooner. 