The green side is to back and the blue side is to lay. Different exchanges may use different colours but back is always on the left and lay always on the right. Usually you will back the event on the bookies site and lay it (blue box) on smarkets. You wont often get an exact match so expect the lay odds to be slightly higher than the bookies price.
I used this guide to help me when I was starting out. http://matchedbettingblog.com/smarkets-guide/#.WFToR_mLTIU