Used Go Blonder spray, can I now add highlight? Help!

I have been using john Frieda go blonder spray on my virgin hair for about 2months, 1-2x's a week. It took it from a med brown to pretty light brown/dark blonde with copper. I now want to add subtle highlight. How can I do this safely, for fear of metallic salts in the product. Their hotline says they have none, but several posts online claim they do and it damaged their hair. If so. How can I remove them from hair? My hair is a bit dry, but I also live in Vegas (hard water and dry air), and I swim a lot. I also never used tools or heat, just wash and go.