The new Gmail inbox classifies email messages according to their importance and assigns different colored  markers next to messages based on various factors like frequency of messages from specific sender, keywords in messages, which messages are more viewed and so on. This concept is called ” Importance Ranking” and markers are called “Importance Markers”.
However if this is not needed then it can easily be turned off.
To do so :
1. Login to Gmail and click on the gear icon at top right corner and select Mail settings.
2. Navigate to Inbox section and in the “Importance markers” region, choose “No markers”.
3. Click on Save for the changes to take place.
All done.