When I click on a specific day on the calendar, what do the different colours on the right mean?

These colours represent the activity for each shift on that day. The colour system used is: 

Green – the shift is filled, and you can see the candidate's name

Light blue – the shift remains unfilled and there are no applications

Dark blue – there has been an application for this shift which needs to be reviewed

Red – a candidate that was previously approved for a shift has subsequently cancelled