Outlook in Microsoft Office 365 allows you to set color category your mails. You can organise your mails by placing them in specific color category for future reference. Mails can be your most viewed, important mail or mail from your colleague or customer.

Step 1: Open the outlook.

Step 2: Click on the categorize option in the ribbon bar.

Step 3: You can choose your color according to your requirement, but in the All categorize option you can also make a different color label or change the color of the label.

Step 4: Like i make a new label and assign the label to the mail.

Step 5: Here you can see the first mail is categorize by a new label.

Step 6: Also, the other mail can be assign by different category or label.