CITROEN DS3 Handbook page 132

Reversing camera
A reversing camera is activated automatically
when reverse gear is engaged.
The colour image is provided in the navigation
screen.
The reversing camera cannot in any
circumstances be a substitute for
vigilance on the part of the driver.
The space between the blue lines represents the
width of your vehicle except for the door mirrors.
The blue lines represent the general direction
of the vehicle.
The red line represents a distance of about
30 cm beyond the edge of your vehicle's
rear bumper. The audible signal becomes
continuous from this limit.
The green lines represent distances of about
1 and 2 metres beyond the edge of your
vehicle's rear bumper.
Clean the reversing camera regularly
using a soft, dry cloth.