i am a PPG certified painter. been certified for over 30 years with PPG.
a couple of things to try.

i am assuming you are painting the color base coat / CC.
yellows and reds are known to darken as they dry. if you clear then to fast they will be light.
i would suggest spraying out the original non match color then bake the base coat 20 min @ 140* before clear coating it. also increase flash time between coats. this can darken problem colors.


if spraying single stage i would suggest baking the spray out card for at least 40 min @140* after proper flash time given. use a fresh spray out card with fresh mixed paint.
i would also suggest giving more flash time between coats. this can darken problem colors.


also yellow based colors are sometimes extremely transparent.

what is the color rating on the spray out card you are using? i would assume they are using a #5 standard gray.
the ones i use for water base coat are rated at G1, 3, 5,6, &7. from white to dark gray.
maybe use a slightly darker sealer color, maybe a #6.. this can give you a darker top color.

maybe try one of those multi striped spray out cards with all of the primer colors. from white to black.
this could be the fastest way to see if you have enough paint on the spray out card. with proper coverage there should be NO difference between all of the stripes of primer if you use enough coats.
with modern tech tints in old formulas, you might need 4 -6 coats for proper coverage instead of 2-3 in the old days.

maybe a combination of darker ground color and another coat will do the trick.

you also could try to thin the color a bit less, thicker paint will give you a darker color. it will dy slower and not lay out like thinner paint, but it could be a better color.

also to contradict the above statement. sometimes yellow tints don't like to dissolve properly, especially in solvent based paint.. you sometimes need to over thin the color to get a good match. this is most common with blue shade green colors that have yellow tint in them. it can also happen in solid colors.



hope this info helps.