At the start of the episode, you see a short clip of Joanna on the cross-trainer in her office, wearing her pink shirt:



However, for the majority of her scenes for the rest of the day (minus the ones at Guy's flat, of course), she's wearing her leopard-print shirt. Including this one, mere minutes after her first scene:





This is because the first clip of her is one that has been reused from episode 2:

Why you clever badger :badger

It happens a lot doesn't it, complete changes of clothes, the office girls do it too, and as discussed somewhere else, they all seem to have just 2 outfits for a series, excluding any special outfits (dressing up for the house party, cheerleading gear, squirrel costumes etc.)