Is there a library of color themes I can use for my app?

Yes, it's the Template library. If you see a color theme you like in any template app, you can copy it.

Use the "Color Theme" copy option and it will copy only the color theme of the template app to your app.

                         

There is also a selection of templates that are designed solely to explore color options. Search the template library using the "colors" tab to see these templates.