Turns out that e-ink displays typically don't mix colors this way. In order to get multiple colors in a single display, they use sort of colored-balls of various densities suspended in a fluid. Then, using the known viscosities of the fluid in the capsules, they can vary the charge of the electrodes very quickly to sort of _drag_ a color up to the top.
But when you do this, you're dragging a _single_ color.
_All_ the colors are in the capsule, but you can only truly pull a single one at a time, a true pigment-like mixing isn't possible. If you pull the Cyan up, you're necessarily losing the Magenta, for example.
If you had a high enough resolution addressable grid, you might be able to put the pixels close enough that you couldn't tell, but you'd still be basically doing sub-pixel color at that point, not truly mixing the various colors at the base level.
Maybe there are other e-ink displays doing something else, but all the ones I've seen are doing some form of this approach.
Here's a fascinating video on how these displays work, and how you can modify the firmware of the driver to get faster refresh rates (with an increased risk of burn-in). https://www.youtube.com/watch?v=MsbiO8EAsGw
An area of research I've heard a bit about for next gen e-paper displays is to use electrowetting[1] which is a different approach that looks intersting
My understanding is that modern printers actually mix those four different inks together on the page to achieve a wide range of colors. That isn't possible with a display where colors have to remain within their own separate (sub)pixels and can't actually mix together.
With a high enough resolution that might not matter, as your eyes would be unable to discern individual subpixels, but this display is only 600×448, and that's with only one subpixel per pixel. (A CMYK display would need 4; maybe slightly less depending on subpixel layout.)
Furthermore, modern sRGB displays can output 255 different brightness values for each subpixel. This display can only output 7 values for each pixel. That further reduces the number of possible colors which would be possible in a CMYK subpixel-based display.
Provided all those issues could be solved though; maybe it would work? There could be other considerations I'm missing. (E.g. Can subtractive color mixing even work with subpixels in the first place?)
But when you do this, you're dragging a _single_ color.
_All_ the colors are in the capsule, but you can only truly pull a single one at a time, a true pigment-like mixing isn't possible. If you pull the Cyan up, you're necessarily losing the Magenta, for example.
If you had a high enough resolution addressable grid, you might be able to put the pixels close enough that you couldn't tell, but you'd still be basically doing sub-pixel color at that point, not truly mixing the various colors at the base level.
Maybe there are other e-ink displays doing something else, but all the ones I've seen are doing some form of this approach.
Here's a fascinating video on how these displays work, and how you can modify the firmware of the driver to get faster refresh rates (with an increased risk of burn-in). https://www.youtube.com/watch?v=MsbiO8EAsGw