Using SetColor() on a Label sets the color as it always has.
Using GetColor() gets the exact same color values as were set.
PROBLEM
When you look at the image, there are hues which are displayed incorrectly, even though GetColor() claims they're the correct color. For example:
0x000000 to 0x060606 all display as 0x000000
0x070707 to 0x111111 all display as 0x0d0d0d
... etc.
I count about 75 hues on each color channel as unavailable, for about a half million unavailable hues.
The above image shows three blocks of X, which have two of the color channels (R, G, B) set to 0x7F, and the remaining channel progressing from 0 to 255.
The left block varies Red, middle varies Green, and right varies Blue.
I've marked up the Red block to show the X glyphs that have the exact same RGB values.
Grayscale behaves the same way:
CAUSE
This is likely to be a side-effect of Slug fonts, since the issue started in that update, so may not be easily resolvable.
It is not an antialiasing effect, as I chose the X characters specifically as they have a 4x5 area in the center of all the same color.
It is not a compression artifact, as it's viewable on the game screen in realtime by using a color picker tool. I've saved the screenshot as PNG to avoid compression artifacts.
WHY IT MATTERS
It doesn't. The unavailable colors probably can't be distinguished from the available ones with the naked eye.
I'd written a script to switch between characters using hotkeys, with an addon displaying the current character ID (1 thru 20) as the red channel of a label showing "." (period). But even for that use case, I can just work around the problem by adding 0x80 to the character ID.
CODE TO REPRODUCE
This code produces the above image.
XML:
https://pastebin.com/mFSKUhM9
LUA:
https://pastebin.com/UtF24x1z
I tried to make this as minimal as possible and still demonstrate the issue.
The code looks spammy, but it's all just the same line repeated 768 times with the different RGB values.
In both files, you can delete all repetitions except the first for a truly minimal test case, though then you'll need to vary the color manually, with a /reloadui for each change. Probably easier to leave it as it is, displaying the whole matrix.