You cannot change glove colors in Wii Sports. The game has zero customization for equipment appearance.
SD:/riivolution/ patches. Requires dumping game, extracting boxing_glove.brres, editing in BrawlBox/Blender, repacking. Complex..png/.dds in Load/Textures/RSP.... Easiest way to get silver gloves.Wii Sports Resort / Club: Also no glove color options. Arms (Switch) has cosmetic gloves — but different game.