Item description:

To accommodate for the undersized, relatively dark monitor of the Game Boy Advance, a variety of colors in Final Fantasy VI Advance exhibit a higher degree of saturation than that of the earlier versions. Consequently, the graphics generally appear brighter, and certain colors diverge significantly from their original specifications. As such, the purpose of this patch, which supports all three versions, is to restore the colors to their former state.

A more detailed description is contained within the document bundled with the release. In addition, a collection of images designed to provide a basis for comparison can be found at the author’s site.