Hey Maxime,
- The call to action button colors are not set from the Appearance page since they use their own.
- If you want to extend the color option, check my answer in this post How to change the image field to include a caption and how to add more color options in the color field. For your case, you should open the Manage Form Display of the Call to Action Button and add more color option in the Background Color and Text Color.
Let me know if that works for you.
Kostas