I agree your mock-up shot looks much better but looking at this on a different screen and settings means there’s going to be colour / brightness differences. Another person might find another colour palette is more suited
I do not mean to discourage you from getting this changed though, as the original colour palette could be bothersome for people with dyslexia or people with some visual deficiency.
PS : Isn’t the Free Software Foundation responsible for GRUB ?
Agree, there will always be differences between screens, even slight variations from the same manufacturer.
To try and help limit discrepancies across display devices, I made GIMP alterations and observations on a correctly calibrated and profiled screen. Considering an icc colour profile isn’t loaded until OS has booted, used VirtualBox to check GRUB and create the screen capture.
On other distributions, I’ve seen GRUB presented with a black background and white text, presuming this is the default, considered the current implementation may be an Ubuntu (MATE) modification, so tried to limit alterations to text colour alone.
Edit:
After further consideration. Most important aspect is an increased contrast range to help with legibility, even with expected brightness and colour variations across various display devices.