Hi!
This is exactly what I am looking for too. Making everything monochrome red is great. But why does it inverts everything at the same time? That should be a separate thing. All my icons look odd, all shadows are wrong, photos are useless, faces cannot be recognised, basically everything looks bad. So instinctively I've tried to use MacOS built in inversion feature to "fix" it, but unfortunately macOS inverts the image after it is tinted red by f.lux, so in inversion mode it becomes cyan. (My brain froze when I switched. I didn't expect the amount of blue light into my face.)
I'd assume it could be very hard or almost impossible to make f.lux filters come after OS level filters, so it'll never play nicely with macOS built in inversion feature I think.
But that should not mean that the red filter should only be available with inverted luminance. Can we gat a mode / option to use red filter without flipping dark and light parts of the screen please?