Nail Issues

After removing my 'toxic trio free' polish from my toes I discovered a dark patch, three years ago, and again last year. This year I removed the colored polish to find dark patches on three nails. After the first occurrence, I left it unpolished but the nail did not clear until the peroxide was used. Each time I soaked cotton in a strong peroxide solution, and held it on by making toe covers from a surgical glove. One application does it. This treatment does cause some pain for a couple hours, but it's worth it to have clear nails after only one application. So, for me, the remedy is so simple, but what I'm wondering about is why after 'colored' polish the nails darken to a brown/red color. The pale colors didn't seem to cause a problem.