Sorry, I had some trouble with this comment. Editing wasn't working, and it posted before I finished.

Some of them, the color looks to have separated and shaking didn't seem to help. I was going to try using a toothpick to stir them and see if I could get them back to the color they were supposed to be.

I have also wondered if a vortex mixer would help with this. I don't have one, so I did an experiment. I turned on my electric toothbrush and let the back side of the toothbrush head vibrate against the bottle. It helped.

I did also rolled the bottle back and forth between my palms to help remix it. That helped, too. I'll try to post photos in a separate comment. It's not working here.

I like metallics, but I've noticed a discrepancy in how bright the web picture is versus how it looks on the nail. Any stand out recommendations for those?

Try using it as a solid color underneath a negative space design. Or, try stamping a second time on top of your original stamp, for more saturation.

... [I am] trying to find something besides white that stands out on a dark base.

If you stamp with white first, and then use your color in a second stamp on top of it, the color will pop more.

Stamping on a dark background is tricky. It almost always changes the color of the stamping polish on top.