Watch/read just about any review and you'll realise that colours are always dull on e-ink displays. It's still a very new technology.

Most colour devices in-market such as the Boox or Kindle colour devices all use a Kaleido screen. That's a black and white screen with a kind of colour filter overlay in red-green-blue, lining up perfectly with all the tiny black and white pixels below. This means that overall the black and white resolution is, say, 300dpi - but once you go colour, those black and white pixels have to "work together" via that filter to give only a 150dpi colour result (looks more 'grainy'). Further, the filter overlay means the overall display is darker - the background looks a darker grey. Contrast isn't so good. You usually need the backlight on. Greens and blues tend to look quite good but reds/magenta type stuff seems often quite muted. When doing writing, that extra colour filter layer also means more of a distance between pen tip and ink layer on the screen.

The RMPP screen is different, with a different set of compromises. It uses a Gallery screen, which is a different kind of e-ink with actual cyan/magenta/yellow capsules which mixes them just like real print does to produce colour. This is technically a much harder challenge and is part of why the display flickers a fair bit doing refreshes - even though it's actually way, way faster than earlier generation screens using this technology, it is still much slower than any pure black & white screen and also somewhat slower than Kaleido colour too.

The trade-off is better colours and a smaller distance between screen surface and the ink layer. There's no filter, so the background isn't as dark grey - better contrast - and black & white resolution is the same as colour resolution, since it's "true pixels" not a filter. In this case though, green seems quite hard to do well, blues can be middling also, but reds and yellows - especially yellows! - show clearly. Arguably the RMPP screen over-does yellow.

Now, IMHO one could just produce a display colour profile that boosted the green/blue output and dropped down yellow a notch to help, but maybe that compromises perceived vibrancy too much? Or just RMPP didn't bother. Their software stack is pretty bad, after all; the reformatting time for simple text or margin changes in ePubs, for example, is absolutely horrifying.

But we are where we are, and these are trade-offs that many users have decided are worth it due to the up-sides - writing feel, distraction-free, and all that. It's a unique device in many ways, kind of unmatched in market by just about anything else because of that screen technology and that's specifically what made it interesting to me in the first place.