Ive used halfords and holts touch up paint pens, mixed colours to match the
silvery blue grey on some Shimano SLX crank arms, and mixed to match XTR mechs.
Cyan blue paint touch up from Ebay and plenty of others, all are car paint touch up products
which all work good for my needs, have assorted clear coates in satin, gloss if the sheen needs to match.
Have used model paint enamals and nail varnish on some when struggling to get exact match in
car paint pens, just experiment on a tile surface and mix away till you get the match, a bit of cellulose thinners
to keep things from drying and getting to thick, if it doesnt work then clean away and start again.
Stick with like for like when mixing to avoid reaction, bubbles, rippling, cracking etc.