This theme uses two green colors and these colors are spread over several CSS properties.
You can just use any text editor to edit style.css and use search and replace functionality to change all these color codes.
Look of for these HEX color codes and replace them with your own color codes – #439f55, #57ad68.
For other questions please look into: http://colorawesomeness.com/themes/support/travelify/ since most of the theme related questions area already answered there.
It’s a beautiful theme I’ve ever seen.