The bug
It is not possible to change how the bundle bar looks via a resource pack - the effect has been generated purely by code ever since bundles were implemented in 1.17.
This (and other recent reports) may count as feature requests, but I'm not sure where the line between that and genuine issues even is, especially given recent changes which have made previously-hardcoded graphics such as the pause menu background, text fields and scroll bars retexturable.
How to fix
Introduce texture files for the bundle bar - one texture for the background alongside textures for each progress iteration (1 to 14 pixels) would suffice. Ideally these should be at least 16x16 so that the bar can be positioned anywhere desired within the slot. There could alternatively only be one progress texture which would be truncated by the game, although this limits resource pack freedom.
Fixing this could also fix issues like MC-270261.
Linked issues
relates to 1
Attachments
Comments 3
For reference, here's the hardcoded color from BundleItem.java:
private static final int ITEM_BAR_COLOR = MathHelper.packRgb(0.4F, 0.4F, 1.0F);
This also affects 1.20.5 pre-release 1.
Can confirm. It's likely possible to modify by altering the vanilla position_color.fsh shader, but from my own testing I could not get it to work properly.