Minecraft 1.13-pre2, Minecraft 1.13-pre3, Minecraft 1.13-pre4, Minecraft 1.13-pre5
This seems to be related to
MC-5206 although italics seem to work.
This appears to be happening because the item rarity colour replaces the colour of the first text component.
Run the command:
And look at the tooltip in the inventory, then look at the item when you switch to the slot in the hotbar.
Comparing this to spawning in a dragon egg with the same command:
You can see that the colour has been overridden by the rarity colour.