Someone posted his Inspire with a bunch of stick-on decals and sent it in for service under warranty and he got his same one back with the decals against a refurb. Something to consider. Metallic paint might effect the signal to a degree but don't know for certain.
If DJI wanted to deny a warranty for paint job, they likely could due to this exclusion in their warranty: "Damage caused by unauthorized modification, disassembly, or shell opening not in accordance with official instructions or manuals." However, if out of warranty it might not matter if you pay to fix it.