How to do I customize the colors used in the calendar panel?

The colors for the calendar panel are pulled from Calendar.app, then the closest match in Status Board’s color palette is applied. Changing the the colors in the iOS calendar also changes them in Status Board.

I am not able to change the color of my Google Calendar.

Google Calendar colors are determined from the settings on Google’s site. You’ll need to log in to your account and change the settings there.