Which CTA works best?
Generally speaking, green and orange buttons are reported to perform best. Ultimately though it will depend on your site design, as contrasting colors work best to make striking buttons that stand out. You wouldn't want a green CTA button on a green background.
What is a good CTA?
Effective CTA copy uses strong action verbs to clearly communicate your offer. Sometimes the most effective CTAs are also the most simple. For instance, a CTA that says "download now" tells the user that they can download related materials just by clicking on your button.