Depends on the bike/era.......and what type of paint you're using.
I'd say, humbrol metal coat polished ally and flat ally for enamel. ..............They dry quick, so brush paint them otherwise it will gum up your
airbrush, leave to harden/gas out for a couple of hours before polishing up with a cotton bud.
For acrylics, I'd use alclad range of paints.
The above is OK for the older style of motorcycle ie 70s to late 80s. For more recent motorbikes with black engines, I would use black satin paint from humbrol, Alclad, vallejo and any of the other mainstream paint makers.
P.S Welcome to the forum