I have Cambria Britannica on my cabs which is white with some grey. It shows much more on the white. Can't really see it on the grey.

My seam is at the sink and it's fine. I don't see this is a problem.

Color doesn't affect the adhesive color match; just make sure your installer isn't color blind (seriously) and doesn't add too much catalyst to speed up set times which can turn white adhesives yellow.

Seams can be placed over sinks, look nice, and be serviceable for years. More importantly is leveling tops with non-compressive (not wood) shims to keep pressure off such delicate joints.

Too much catalyst causes yellowing.

A little white tint helps a bit. (Not my job.)

I am wondering if the seam gets dirty over time if it is over sink. I can imagine food and frequent use will darken the seam a bit over time.

Your seam can be the width of a hair and almost invisible with quartz. You shouldn't have chippy edges like you might with a granite with crystals, especially if your quartz is marble-like or other quiet pattern that is basically resin and fine sand particles. They can also use a seam setter that electronically pulls the two slabs together. That's what I attribute my very tight seam to, but I'm not sure if this is an option with a sink seam where there's less of the slab to grip. The colorant is the least of your worries as that can be adjusted.

Ask to see samples of seams they've done. Also ask to see the slab and how the template will be placed. Quartz can have unsightly resin pools (voids where there's no quartz pieces) that you'll want to template around. Cambria seems to be especially prone to resin pooling. Also your slab may be very different than the sample.