One of our users is red-green colorblind, and has difficulty reading/understanding e-mails with diffs highlighted in red and green. They provided this snippet of edited diff description as an example where they couldn't tell the added parts from the removed parts:
{F2298567}
The user said that it was especially difficult to perceive the difference between the red and green, because both were similar in brightness and pale.
The color contrast display settings don't seem to affect the colors used in e-mails, either.