FWIW, Space Gray touch up paint is fairly close to the 18" wheels. The base coat looks a little lighter than the original paint, but it gets a little darker and closer in tone if you add the clear on top. From some angles it is almost perfect, from others it is a little bit lighter/more silvery. Not sure if that is due to reflectivity. Might be able to play with how much metallic flakes get applied to make it lighter/darker (it is dark gray paint with metallic particles that kind of "float" to the top as it sets - it looks too dark on the brush, but turns out OK).
Golden Gate BMWCCA - Car Control Clinics
'13 C4S/'12 328i/'04 GT3
Past: '09 M3 DCT/'03 M3 SMG/'95 M3... missing an E30 I suppose...