One thought: Limitations in the holographic display would support the need for flat-panel displays.
Another thought: In real life, if a monochromatic holographic display were developed, I'd be very surprised if monochromatic displays at various wavelengths could not be developed that were suitable for red, green, and blue color primaries. Further, I'd think that simply superimposing the displays would naturally result in RGB color. But perhaps the resolution is so accurate in the individual monochromatic displays that precisely
superimposing two primaries, much less three, is problematic. Maybe it's hard to get them perfectly aligned, so it's usually preferable to operate in monochromatic mode. And maybe when color is needed, two primaries work better than three.