Just go to Halfords or an auto factor and buy some. I’d prime it first using a primer even though you are painting on to a painted surface. Gives better adhesion.I'm not bothered about matching as I would be doing the full length, just what sort of paint I.e. airfix, acrylic, plastic paint etc.
Another one for Paintnuts, they matched the paint from the registration No, it was a good match.I used Paintnuts to spray the rear wing of my daughters focus it was a great match. She got a good trade in. On my own silver Forester I bought a tin of touch up from paintnuts again very good match.
Not sure how you obtain match for that part as cars paintcode is generally for main colour. I’d probably go for spraying the whole length of trim and as long as it’s not near another section of the OEM color no one will see the difference v