Most Megaphone pages will let you pick from color variations to help the page better match your message.
For instance, if you created a "Default" colored Webinarly page, you might choose to brighten it up later by picking the "Colorful" setting.
In the left sidebar of the page editor, click "Colors":
Change from "Default" to "Colorful" in the dropdown and click "Save"
Note: Your new colors are in use instantly when you save. Enjoy!