The lock, login and logout screens are coded in such a way that they utilize the complementary colors of a particular color scheme.
Breeze and Breeze Dark have been adjusted to work normally under this condition. Normally = white text and a dimmed background in both cases.
It is still, however, possible to get alternative colors with some desktop themes and color schemes.
This results in black text and the wallpaper being brightened instead of dimmed.
The end result is not very attractive, particulary in the case of the lock and login screens::
A usability issue also presents itself in the sense that we can't (shouldn't) put shadows behind black text (see D24997)
It also seems this modularity feature is not that modular. From my testing, text is always either white or black, and not some custom color.