Activities and Activity Templates are assigned a Difficulty Level that determines how they are colored when applied to a Calendar:

Easy - These Activities will be the lightest shades of grey/blue/green
​Medium - These Activities will a medium shade of grey/blue/green
​Hard - These Activities will be the darkest shades of grey/blue/green

The resulting Calendar is colored as a "heat map" where one can easily see where easy, medium and difficult days are scheduled (along with Activity Completion, goals and Personal Records). 

You can read more about Activity colors here.