Football League regulation say that clubs MUST register thier Home kit during the close season and that kit MUST be worn in ALL Home games, with one exception where clubs can wear thier registered Away kit or 3rd kit (If they have one) in one Home League match without having to ask permission.
So as Fulham are likely to wear thier White Home Kit tomorrow because thier registered away kit clashes with our Red we could wear the Blue tomorrow.
Another Football League rules gives the FL Board the "Final Decision" over ALL matters relating to thier rules so the Club could ask for permission to change back to Blue immediately. It would be a massive decision for the FL Board to say NO to that request because it would go against the wishes of the clubs fanbase. Lots of negative publicity for the FL if they take the wrong stance.
In respect of the FA Cup games I believe clubs are required to inform each other of thier colours 5 days or so before the game so that should be an easier situation in respect of the Reading game.