strategic color definitions

Every color you use should have a reason behind it. You can have colors for doing something as well as colors for going somewhere.

Usually your app will have a single distinct, full saturation color for a primary call to action as well as a lower contrast color for a secondary call to action.

These colors can be brand colors or not and might compliment each other.

In addition to your CTA colors, there should be structural colors that are more neutral and build the basis for your application. In many user content generated sites structural colors are off-white or light grey as a lot of color comes through the content that the user adds to the site.