I have designed an app that contains 32 screens, but now I'd like to see it with a different color scheme. Can we get a way to switch palettes quickly or a global color select (i.e. like Illustrator) ?
We don't have a feature like that yet. We're hard at work on adding support for reusable symbols -- that will make it easier to edit a color in one place and have all the identical/similar copies of that object update too.
If you have a color theme where one color is used on many entirely different visual elements though, then symbols may not help you as much. If you're more interested in that, please add your vote to Global Style/Color Edits – Adobe XD UserVoice.