Got a ticket today. I thought I was able to pay & display (which I did) but the signs were confusing. I looked at the ticket and it stated that my car's colour was grey, when in fact it is dark blue. Is this grounds to challenge the ticket?
If you explain the reasons why you thought it was Pay & Display and show them the ticket you purchased, with best intentions, you should get away with it.
If that fails offer to let them take you to court, but point out that the ticket is technically incorrect with regard to the colour of your car, which will cause it to be thrown out in court anyway.