Background colours in emails are generally not recommended. Most background colours in email are not displayed by various email clients, such as Gmail and some versions of Microsoft Outlook.
When you create a new mail, you can play around with template design, colours and palettes in step 1 of the email creation wizard. The Page background colour picker allows you to change the background that appears behind body of the email. The Content background colour picker allows you to change the colour that appears behind your text.