My bikes are too dated for OEM touchup paint to be available.
I've had pretty good luck with Hobby Town for Humbrol and Testor's enamels. If you get the balance of colors right (red, blue green, yellow), mixing a little white makes it lighter and a little black makes it darker.
Still, almost impossible to get it perfect and in the end, the shape of what you put down affects the bright/dark. After it's good and dry, I'll use a little 4000-grit sandpaper followed by Meguiar's No. 7 or polishing compound, then wax.