Light grey and grey banners are named "light gray" and "gray", even when using English UK.

Steps to reproduce:

1. Set language to "English (UK)".
2. Create a world.
3. Mine two logs.
4. Convert all the logs into planks,
5. and two of the planks to sticks.
6. Kill six sheeples, all of the same colour.
7. Eat the flesh of your dead sheeples. (optional.)
8. Press four of your planks into a crafting table.
9. Check if your wool is grey or light grey. If it is either, skip to step 15.
10A. Kill three squid and a few skeletons.
10B. Collect six azure bluets.
10C. Collect six ox-eye daisies.
10D. Collect six white tulips.
11. Figure out how to make grey or light grey dye with the stuff you got.
12. Mix each wool block with a dye.
13. Put a stick in the lower middle slot of the table,
14. and drag your wool around the two top columns of it.
15. Hover over the output slot of the table.
16. ???
17. Profit!