On e-ink screens, the grey pieces look more different in color from the blue pieces than the red pieces do, though they still look closer in color to the blue pieces than they do to the white pieces. I tested both versions on my Likebook Mars and my Kindle Scribe, and I compared a different version on each device side-by-side, then switched them. Based on these tests, I would recommend grey over red, and I would recommend a lighter grey over the current shade of grey.
On e-ink screens, the grey pieces look more different in color from the blue pieces than the red pieces do, though they still look closer in color to the blue pieces than they do to the white pieces. I tested both versions on my Likebook Mars and my Kindle Scribe, and I compared a different version on each device side-by-side, then switched them. Based on these tests, I would recommend grey over red, and I would recommend a lighter grey over the current shade of grey.