Do seems show worse on light color quartz counter than dark?
That might influence my cambria quartz counter color choice.
Probably, but only because the seam, even if done well in a white quartz counter, will gather dust and gunk, which will turn dark. Eventually, this will stand out against a white counter while it would blend into a dark counter, whose seam is matched dark to begin with. Same idea as with tile and white vs. dark grout.
I ave a nice tight seam in my white Caesarstone Blizzard counter. I barely notice it, but I take care to keep it clean. There was a recent post about this, where I posted pics of my seam.
Here is a link that might be useful: Another Seam thread
Shouldn't actually make any difference, especially with quartz which has a uniform pattern. A nice, tight seam with a color matched glue (epoxy?) should be nearly invisible regardless of the light/dark issues. If you're concerned about color matching, get the details from the installers before you commit.
It's an art. I have a light colored, uniform quartz. They mix the epoxy right there in your home and try to get as close as possible. They did a great job on my seam, but they scratched my counter cleaning it with Barkeeper's Friend, so they replaced it. The seam on the second counter is color-matched even better and almost disappears.
I just got white quartz installed yesterday. I have 2 seams in my island due to a post, and they are not very noticeable. One can certainly find them if you look, but they aren't visible at first glance.
I had black granite in my last kitchen, and the seam was much more obvious.