The color set in Preferences -> Display -> Formatting -> "When displaying quoted plain text messages: 'Color:'" is not used for displaying the quoted plain text in a plain text reply. Instead, the default of '#0000ff' (blue) is used instead of the user provided setting in mail.citation_color (here #5182E4). In the provided screenshot, you will see that the setting box for choosing the color in Preferences -> Display -> Formatting -> "When displaying quoted plain text messages: 'Color:'" DOES properly reflect the mail.citation_color setting -- it is just not properly used by the compose window when hitting 'reply' In the screenshot, you can see that the quoted plain text in the compose window is #0000ff despite the user selected color. On dark background, this makes the quoted text very difficult to read. However the colors are set in the compose window for reply text, it should be set to the mail.citation_color and not $0000ff

Apparently that preference only applies to displayed quoted text but isn't effective in the composition window. The example shown is a good one given that the black default background causes the dark blue of the quote to show up poorly. Other settings in the Display tab (e.g., fonts and sizes) apply to the composition window as well, thus I'd tend to confirm this in order to keep it consistent.