Activities are color coded on Sixcycle Calendars based on two factors:

Past or Future
Activities in the past will be gray or green, depending on whether they have been marked Completed (green means they have been Completed). Activities in the future are blue. Goals will always be red and Personal Record days will always be gold.

Difficulty
When an Activity is created, the author of that Activity assigns it a Difficulty level. This determines the intensity/shade of gray, green or blue, as appropriate, with easier days being represented as lighter colors and harder days being represented as darker colors. 

The colored bar to the left of the Activity shows the plan with which the Activity is associated. Plan colors are shown to the left of plan names in the Timeline.

Feel free to let us know if you have any questions!