I think because wheels are only 18s, the brakes and callipers look proportionate and not too small. So the painting works.
However, i'd go for this color scheme: black or anthracite rims = yellow or bronze calipers, silver rims - red callipers.
Although personally I'd paint them body color, black or silver (not to stand out).
'07 TiAg E86 Z4 Coupe 3.0si