The out of the box Activites Report only shows activites that are regarding Opportunities and Cases.
The report includes an option to show all. This uses the Activities Detail report which includes colour coding against the activities.
By analysing the rdl file for the report the following sets the colour circle to show:
My interpretation of the colour coding is as follows:
If the scheduled end date is than 7 days from now then green circle
else If the scheduled end date is not specified then green circle
else If the scheduled end date is later than today then amber circle (but by implication of the IF nesting less than 7 days away)
Else red circle