Ignore on trading shop color and NBT tag on shulker box

See the section on shulker boxes in the wiki's 'known issues': https://github.com/Shopkeepers/Shopkeepers-Wiki/wiki/Known-Issues
Basically, Minecraft inserts NBT data when you place and pickup a shulker box. This picked up shulker box will then no longer match a newly created shulker box. This is not something I can change.
If you want the trade to ignore the NBT data, you need to use a shulker box without the NBT data when you setup the trade. This trade will then accept shulker boxes both with or without the NBT data

Shopkeepers adheres to Minecraft's trading / item comparison logic. Since differently colored shulker boxes are different types of items, they are not considered equivalent in regards to trading.