Greetings.
As I was recently trying to texture the assets for this mod, I came along some unpredictabilities regarding the colors this mod uses for its GUI textures. I will list some of my concerns:

1. The 4th sprite in the widgets.png is very confusing. It basically acts as an overlay, but to what degree and what is its function in the first place?
2. After recoloring every sprite, I have discovered that the colors of the advancement header banners, the first 2 sprites on widgets.png, are actually hardcoded and widgets.png does not have any influence on how they look, however they are influenced by the 4th sprite again, to some degree.
3. The first row of the 3 advancement bases (the yellow boxes, for the finished advancements) is also not used, in game they are just the default boxes, the row below in widgets.png, with a filter, with a hardcoded color value.

I hope this was understandable, I'm not that good as writing texts, but I'd love if you addressed those issues as it would be awesome for texture makers like me! :D

I don't use the colours from the sprites, I recolour them in code. That way they can be set by the player / advancement creator. So I think you only need to do the grey ones on the sheet, plus the black one which is the background for the description.

Hope this helps!

Ah, this makes sense of course! Though my concern is that not many mods use custom colors, and if that's the case, Better Advancements looks worse than vanilla advancements would look, because it doesn't apply the textures defined by the resource pack.
A little feature request: It would be awesome if you applied widgets.png like in vanilla, which would then get overridden by mods using custom colors. That'd really be cool :D

The issue is that to override the colors I need to use the grey ones, and I just apply the default vanilla color in code is easier than having tons of ifs. But I do get the problem, maybe there would be an option to let the resourcepacks decide on the default color

Let me get this on the timeline for a fix, need to figure something good out for it. Something that allows for both customisation and works well with texture packs.

Will likely need a "blank" texture for colouring and "normal" ones for other places.

Full info on how textures should be made can be found on the wiki page