You’re told there is a mistake and asked to identify it. You probably won’t notice it immediately, but once you do… you’ll be kicking yourself.
The logic behind the answer is our brains when our brains know there is an error, they scan for one that isn’t there.
So, a lot of people will immediately look at the multi-coloured numbers to see if that’s where the mistake is.
As all the number are correctly sequenced, we then analyse the colours they’re written in.