Cartographers can never sell light gray banners.

While looking through the files of the game to make a data pack, I noticed that the Cartographer had its trades be biome specific for its banners. They go as the following:

Plains - Brown, Pink, White, Yellow

Desert - Cyan, Gray, Green, Lime, Orange

Savanna - Green, Magenta, Orange, Red

Taiga - Blue, Lime, Pink, Purple

Snowy - Blue, Cyan, Light Blue, Red, White

Jungle - Brown, Green, Yellow

Swamp - Black, Light Blue, Purple

However, there is no code file that allows them to sell light gray banners, effectively making them impossible to get from that specific villager.