In an oven situation, dark color is not as important. When you heat something in the sun, there's a lot of visible-spectrum light, which is why you'd want dark colors.
Probably, most of the available toxic metal content of tiles would be in the glazes, if any. If they are ceramic, there can be traces of many different elements in the ceramic itself, but they're generally not accessible.
If your pizza stone lasted ten years, that's pretty good. Is that brand & model still available? I'd go for that.