I have never pulled thevinyl graphics from a boat, but we just striped the vinyl off our motorhome to put new graphic on. The best way I found was use a blow dryer on a warm day. The vinyl and glue come off together. Mineral spirits is good for removing any remaining residue.
As far as the color difference, that is the work of the Sun, I haven't found anything to change that. The folks that replaced my graphics said it will blend with time.