The Harsh Light Of Day - S4-E3
Corrected entry: When Spike and Harmony are near to the area of the Gem of Amara, Spike tells Harmony that she can't go out anywhere. Harmony is wearing a pink corset top, but when they reach the actual area of the gem, the top changes to a brighter pink halter neck. This happens in a short space of time, so she wouldn't have had time to change.
Correction: She would have had time to change. There is a whole sequence in between the two scenes with Harmony changing clothes that shows Buffy at various points during the day (she checks the answering machine several times). Several hours have probably passed between the two times we see Harmony. It could even be the next day.