Performance wise, having 2 materials on one object is equivalent to have 2 "splitted" objects with 1 material. Usually i only use this for easier maintenance, as editing several objects in a 3d software could be a bit more of work if they're meant to be "welded" Unity can batch together objects with the same material…
Like so ?? Maybe via a shader trick. Using a texture with an marked area (not only one color but color "corection" red / 2.. so there are still some details): and comparing the color to get a "mask" while changing the hie/saturation for this part only (via mix color) like so (here with animated hue/sat) teh coloring…