The reservation schedule allows you to create several bookings that overlap and therefor cause a double booking (or overbooking). Wenn that happens, the two colours of the bookings will create a mixed color during the time of the overbooking.
To quickly view all double bookings in the reservation schedule, you can use the booking list below it. If you can't see the booking list, go to View > Show Booking List.
1. Select the filter option "Double Bookings" from the drop down menu in the booking list.
2. Press the arrow next to the status entry to view the booking in the reservation schedule.
3. Drag or alter the booking in the reservation schedule to resolve the double booking (you can also open the booking by double clicking on it and then change the time frame from there).
The visibility of double bookings depends heavily on the transparency of the bookings themselves. You can adjust those transparency settings under Preferences > Reservation Schedule > Colours. The best visibility of double bookings is achieved when using a transparency of about 40% (you can easily adjust that by pressing the reset button).