The bug
In this pre-release the descriptions of advancements have gotten colors based on their frame (challenge = dark_purple, frame and task = green). These colors should be described in the text-component of the advancement's description, rather than default to a certain color based on the frame.
The fix
Make the descriptions of advancements have a "color" key which sets the color, rather than the frame setting the default color.
Comments 8
This is intentional, it can just be overwritten with the color component anyway.
This is not the only place using automatic colors either (item rarities, enchanted items).
@ FVbico, I’d rather see such a response from a Mojang employee. Might as well mark MC-124458 (a bug you reported) as WAI as well then...
For that one it's the only place it's italic by default (inconsistent); and it's already tagged as "potentially WAI".
There was a bit of discussion about this report and the result was that this is WAI.
Confirmed in 1.13-pre7.