This also applies to Prism mod, I don't know if I should open an issue there too, whenever you use Iceberg and/or Prism this issue appears.

Wynntils is a specialized mod for Wynncraft server, which is an MMORPG type server. One of its features is to add an overlay to items to show their rarity visually. This works fine, except for unidentified box items. These items have one more special thing, they render an icon showing the type of item they will contain. The issue is when you have Iceberg installed (or Prism) then only for those items the highlight and icon will be gone, it will not render.

This is how it looks with just Wynntils:

And this is how it looks with either or both of the mods installed:

I tried to look at the logs if something appears there and there is nothing unless I missed it https://mclo.gs/rrZrFmb

This issue started happening after Wynntils got ported to 1.21 on both Neoforge and Fabric. Previous versions worked fine. As these mods are required by Legendary Tooltips which is very popular among Wynncraft users that use mods, this issue gets reported very commonly.

TextColorMixin, specifically icebergConstructor1 and icebergConstructor2 appear to be the issue as they were not present in the 1.20 version. And the same mixin on Prism (prismConstructor1 and prismConstructor2). It's breaking this function in Wynntils as the colour value is being modified

Encountered the same problem under the same version

Fixed with Wynntils/Wynntils#2956