In this strategy's settings you can connect Smart Lists to colors. Then tasks or projects that match those smart lists are shown with a respectively colored line on the left.
This strategy is great for you when you want to have a lot of visual context and want to quickly identify tasks and projects from one or more smart lists.
Creating a new colored line
By going to the settings of the strategy you can add a new line by clicking on "+ Add Line" button. Then you choose which smart list you want to connect it to and choose the color of the line.
Red line for due items workflow snippet is a good idea on how to use this strategy.