The Mac will be just fine without this metallic paint on the inside. The only thing you have done is make the Mac hideous on the inside, which you can't even see once it's put together! No harm done. The paint reduces interference. So if you have it right next to another monitor, there may be more interference now.
If it really bothers you, you could mask it all off and spray it with chrome spray paint. You may even find a flat metallic paint that would match better to the original.