Both of their orders are wrong.

The first person’s doughnut (pictured) has a single grain on it, so instead of getting “hundreds and thousands”, they’ve got a “one”.

The second person’s order has been messed up in a more surreal way - instead of rainbow sprinkles, they’ve received a single sprinkle that isn’t actually any of the colours of the rainbow.

I think it may have been over-complicated by having two punchlines in the last panel. Either of them could have stood on their own but having both on the last panel takes some of the impact out of the punch. Also, having two separate strips about this shop messing up orders would help establish this location in the comic’s world.

The joke is that hundreds & thousands wouldn’t really be “Hundreds & Thousands” if there was one, and the same goes for rainbow sprinkles wouldn’t be very rainbow if there was one, the Ultraviolet was just Cartoon silliness, the to jokes at once was because Hundreds & Thousands is a more British term to my knowledge so I did both so Anyone could get the joke.