You can make 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.
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.
To color-code messages manually, select the messages in the message list, choose Format > Show Colors, then select a color. Mail changes the background color of the selected messages in the list.
The color-coding is visible only on your Mac; it’s not shown when you look at your messages on your other devices.