1. A storage medium, comprising: audio-visual data; and text-based subtitle data to provide subtitles of the audio-visual data, wherein the text-based subtitle data comprises a plurality of dialog presentation units and a dialog style unit defining a set of output styles to be applied to the dialog presentation units, and each dialog presentation unit comprises dialog text information, time information indicating a time for the dialog text information to be output, palette information defining colors to be applied to the dialog text information, and a color update flag indicating whether only the palette information has changed as compared with a graphical composition of a previous dialog presentation unit.