Figured something out...
*EDIT: BELOW IS INACCURATE. SEE POST 30
https://forums.macrumors.com/posts/17463746/
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I've been playing around trying to figure out exactly where the OS is reading the image to determine whether or not to display white or black text.
I performed these tests on the home screen.
Using a 1136x640 blank canvas I added complete white. The home screen shows black text. Then pixel row by pixel row I added black to the image from the bottom up. Once I reached row 236px the text changed to white.
So far clarification, if you take a 1136x640 canvas and add a black mask with the dimensions 235x640 to the BOTTOM of the image, you will get black text on the home screen.
Here are my examples:
Fist screen shot shows a white background with a black bottom mask of 236x640 (1 pixel too high) and you will see the text is white with shadows:
Now with the black bottom mask correctly sized at 235x640 you can see the home screen text is black:
Here are the wallpapers I used:
235x1136

236x1136
**EDIT**
Played around a little more. As it turns out, the actual location of the black mask is irrelevant. You can place it anywhere. The important part is that it is at least 150,400 pixels worth of black or 36.7%
In other words, in order to display black text the wallpaper needs to be at least 63.3% white or have 576,640 white pixels. Just a rough estimate.
