You can set different background colors for emails in the folder list using email flags.

Firstly you need to enable the background color for a flag. Open the flag selection screen using the flag icon on an email or in the action menu. Then edit the settings for a flag (View) You can also edit flags in the App Settings - Email Appearance screen.

Tap on "Use as Email Background Color" and choose a color for the background. The background color not necessarily the same setting as the flag color.

Now all emails that use that flag will have the flag background color in the email list view.