All projects come with a ready-to-go package of frequently used emails in form of email templates.
All of these templates match colors you picked up when creating the project. You can always change template design preferences in Templates > Tweak design. Learn more about editing the template design.
Existing templates can be edited by clicking the pen icon next to a template name. After that, you'll be taken to a template editor, where you can fine-tune pretty much everything. For more detailed information about the template editor, read how to create new template with Sidemail's template editor guide.