The only problem with this question is that it is UTTER RUBBISH! Is there anyone out there who is really so stupid as to think that there would be FA rules relating to the above. And why Preston North End anyway? Why not The Wanderers, who were the first winners of the FA Cup? Surely that would be a more likely rule? And what exactly does "playing in the Royal County" have to do with anything? Name one reason why occupying the same county as a group of German parasites should have anything to do with the colour shirts that a football side might wear.
There is a theory that this particularly stupid question goes back to Reading's away trip to Brighton & Hove Albion in the 1992/93 season. That year, the Reading marketing people introduced the new first team kit of blue and white hoops, to general acclaim. However, they were also exceptionally stupid and created a reserve strip of yellow and blue hoops. When they played away to a side with blue in their shirts, and so had to wear their change strip, quite a few referees took one look at the Reading away shirts and said that the game couldn't possibly take place like that.
Normally, this meant that Reading had to wear the home side's away shirt - this happened at Wigan Athletic and Huddersfield Town, for example. At Brighton, for whatever reason, it was the home side who changed into their away shirt. Reading played in the blue and white hoops, and it appears that the legend of the question was born.
Quite where Preston North End came into it, I have no idea.
someone told me he had heard a version of the answer which included Notts County on the basis that they were the oldest League club and founder members. Clearly, though, this is still rubbish.
somebody else told me that until the early 1970s Arsenal always wore their change strip in games against Nottingham Forest, whether playing at home or away. This was as a mark of respect for the fact that it was Forest who donated Royal Arsenal a set of red shirts in 1887 when the London club was formed. Should this question ever appear but replacing either Preston or Reading with Nottingham Forest, then this tale is likely to
be the reason why.