I need to touch up some very glossy black metal trim on a car.

I am also a BMW owner. The OEM BMW touch-up kit contains 2 spray cans - base color coat & clear coat. BMW has a OEM gloss black paint, code 668. This touch-up kit might work for you.

 

When I can't get the paint code, I go to my local NAPA to have them custom mix it; they even do touch-up paint sizes. But not every NAPA has paint capabilities, so you may have to look for an automotive PPG store

 

Don't you have access to a body-shop that can manually mix a "cup" of paint. The guy who did all my car, will just have a look at the car and will steer a few cc of paint, adding a drop of yellow in order to match my ageing paint. By contrast if the paint is two complex and the tone can only be achieved through the colour match equipment, there is a minimum volume which makes the sample more expensive. This should not be necessary with a non metallic black.

If the shop owner is nice he will provide a slow hardener...

 

I agree that it depends on the relation you've got with the professional...

You may try dedicated automotive paint supply: "Automotive Art" is locally distributed in your area. Never tried them, but judging by their product line-up their colour-matching solutions are comprehensive and could meet the need of a DIYer.

As a last resort you can match the paint yourself: a black paint that has a visible tone variation is usually leaning toward blue or yellow(brown), by contrast a more "neutral" black is usually boosted with red. I would presume yours has a red cast (?). Assuming you are mixing paint in a coffee-cup size, a single drop of yellow / blue / red should be enough. You should also make the visual match under natural daylight as the outside light has a strong blue cast which highlights yellow errors.

If the above mentioned automotive paint is too much an expense for a touch up, I would try hobby grade paint for the base coat together with a proper automotive product for the final clear coat.

For the pea-sized chip, a small set of Tamiya putty filler and polishing compound will make your life much easier.

 

Gunze is actually the maker of Tamiya products. You'd better use the "Tamiya Basic Putty". This single part putty is "thick primer packaged in a toothpaste tube". The consistency and texture is similar to Nutella. Contains lots of lacquer based adhesive, does not crack, dries in 15 seconds, easy to sand. "perfect Bondo".

 

Nail polish

 

. BMW also has an OEM set of 2 paint sticks for all their paint codes.

 

The problem here is that paint do age, and there is an important colour drift. So even if you have the exact colour code, this will not be a proper match. Moreover nowadays paint will be water based, not solvent based so it's not going to be the "same" paint.

Mixing paint (or nail polish as suggested) is probably the way to go. Maybe a painter not necessarily from the automotive industry could also help.