I base the color palette for a project on how the client (or I) answers a few questions:

1. What do you want the passerby to see? What do you want to communicate about this project at every user touch-point?
2. How are you trying to say it? Angry, Friendly, Stirringly, etc.
3. What do you already have that we can build off of?

Based on the answers to these questions, I create a palette that aligns with the client's goals;