This is about Color in the human-interface-guidelines from apple:
Make sure your app’s colors work well in both light and dark
appearance modes. With the exception of watchOS, which always uses a
pure black background, the platforms offer a dark alternative to the
default light appearance. Dark Mode uses a darker color palette for
all screens, views, menus, and controls, and can increase vibrancy — a
subtle effect that dynamically blends foreground and background colors
— to make foreground content stand out against darker backgrounds.
System colors automatically support both appearances; if you use a
custom color, you need to supply both light and dark variants. For
guidance, see Dark Mode.