You can make email messages easier to spot by using a rule to automatically color-code them. For example, messages with a high priority could have a red background.
In the Mail app on your Mac, choose Mail > Preferences, then click Rules.
Click Add Rule.
Choose whether any or all of the conditions have to be met.
Click the conditions pop-up menu to set the conditions for color-coding. For example, choose “Priority is high.”
Click the “Perform the following actions” pop-up menus, choose “Set Color of Message,” then choose either “of background” or “of text.”
Choose a color from the pop-up menu, or choose Other and use the Colors window to select a color.