monochrome video?

[citius]

It's a combination of the viscosity of the fluid containing the color (pigment) spheres, the charge applied across it, the drag force entailed by its motion, etc.

I would say that it's a tradeoff between not just persistence and refresh speed, but also energy storage and leakage currents in the fluid too to maintain the position; both play a role in making sure that the spheres are maintained.