FWIW, iOS already allows color filtering to display only red (or any other color).
I believe macOS does too; and then there's the venerable f.lux, which not only has warmer tinting but also "Darkroom mode" (red only).
I believe macOS does too; and then there's the venerable f.lux, which not only has warmer tinting but also "Darkroom mode" (red only).
However, there are two technical points, and one non-technical point, I would like to reply with.
First, as in my other comment, software filtering mostly reduces the blue and green light emitted, but does not eliminate it.
Second, these blue and green pixels become "useless". A monochromatic red-pixel display could have higher resolutions, or lower complexity and power use.
Third, and the biggest reason (in my opinion), is that it is not optional. A red-pixel only display, does not allow you to change "the warmness" or adjust the colors. Instead it forces its own color mode.