The only thing that worries me about painting side skirts is that they are not smooth. Skirts that are intended to be painted are smooth whilst black ones are textured. This will affect the final paint finish, unless a "filler primer" is used. Perhaps find someone who has had it done satisfactorily and use the same bodyshop.
Halfords.. they lie like cheap Japanese watch! they _can_ order matched spray paints & touchup sticks if they want to. I got one a while ago (it took a couple of branches before anyone admited to it) & the match was spot on..
According to the bible : New Testament (the parts CD) Sport pack 155's came with different side skirts. They were smooth as opposed to textured. Mine have been painted, and also the tops of the bumpers. Looks neat I think.