The bug

Most Survival-obtainable items with non-white name colours have this aspect defined through the rarity system. Ominous banners do not, which is the root cause for some inconsistent behaviour when compared to items using rarity:

• MC-132201: items with rarity don't display name colours in shulker box tooltips; ominous banners correctly do
• MC-249067: ominous banners lose their name colour when renamed, which does not happen for items with rarity

Steps to Reproduce

• Obtain an Ominous Banner
• Run /data get entity @s SelectedItem while holding the Ominous Banner

Observed Behavior

The color field is used in the item_name component to specify the Ominous Banner's name's color.

Expected Behavior

The rarity component is used to specify the Ominous Banner's name's color.

How to fix

Introduce a dedicated rarity for the ominous banner and apply it.