believe it or not, many common enamels will work, if its just a scratch fill.
Ive had real good luck mixing/matching for various touch up on this & that, with testors model paint. plus its stupid cheap and easy avail, so it cost next to nothing to play around with. They have several military/aircraft gray's that are/can be made to be darn close.
As 18widecat said, use model enamel. Take a well-lit picture with you of the caliper and go to a local model shop. They will have hundreds of colors to pick from, you should be able to find a perfect match without even having to mix. I found a perfect match to the factory red Brembos that I use on mine. clean the area with alcohol or brake cleaner to remove any oils or dirt and use a small model paintbrush to touch up any scratches. So far the few little marks I touched up are still looking great 3 to 4 months later. You would have to get out a magnifying glass and look very closely to locate them.