I ordered the hightlight metallic paint (color code: 18G) for my miata.  It looked like it matches when I did the color test on the paper, but when I applied it on to the chipped area, the paint looks lighter then the surrounding paint.  I thought that it would dull up a liitle and matches when I sand it down to even it out, but it still have a noticeable lighter shade.  Is there anything I can do to make the touchup paint a little darker?

Thanks

Metallic paints should be blended. See https://www.paintscratch.com/touch-up-paint-directions/blend.htm. If you are brushing the paint, it's very difficult to match the exact orientation of the existing metallic flakes and it can look both darker and lighter.