Is It Possible To Change Card Text Color Based On What You Click On Pie Chart?

At this time, visuals only know about the report theme colour palette, and not about each other's colours, so even with custom visuals, you're not going to be able to understand the colour of a selection in another visual, just the filter context in the current page (if you have selected a specific data point).

I think that this is an ideal use case for expression-based formatting, which is planned to go into public preview in April (although the release notes are indicative). In this future-state, I imagine that this would work by setting up a measure to handle colour assignment and then this measure (or the DAX behind it) can be used anywhere that you need it (colour assignment in the pie chart; background assignment in the card).