[quote name='Temporaryscars']Is a gray shirt with gray jeans a bad idea?[/QUOTE]
Not unless they are the EXACT same shade of grey AND you have something with color to accessorize. If it's a grey button down, a loose red tie will work, otherwise, go with a red or blue belt.
Don't go too bright - save that for all-black. The grey is muted, so go with a slightly muted color, as well.