Some Referees carry both Red and Yellow Cards in the same pocket.
Yellow Cards are generally rectangular in shape whilst Red Cards are elliptical (sort of round)
This facilitates the Ref getting the correct card out of his pocket without having to get them both out and look, particularly useful when a quick dismissal may just defuse a situation (e.g. the sending off of the Havant & Waterlooville player in the first match v Swansea).
I (and a number of other Referees at many, many levels) have a Yellow in my Left Pocket (L= Leave them on), a Red in my Right Pocket and another Red in either my back pocket or my shirt pocket in addition to the cards in my 'book'
Hope this answered your question