It is a program which adjusts the color temperature of your screen with GTK+ integration. Think about daylight and night light difference. From the developer's page: http://jonls.dk/redshift/

Redshift adjusts the color temperature of your screen according to your surroundings.

It has been in the repositories since Precise Pangolin 12.04, and you can install redshift-gtk additionnally to it.

Redshift-gtk is a simple user interface. Once installed you will find it in the accessories menus. It is not installed as a dependency and isn't even a recommended package, for the time being.

It is used to start redshift, and once started, it sits in the tray to allow enabling and disabling redshift, suspend it for a while, or quit. This is all it does, and it does not provide a configuration of any kind. The man and help commands are not very helpful either, especially on the syntax needed for the latitude and longitude.

So here is an example of a working configuration, which you can adapt to your needs. Name it redshift.conf and place it in your $HOME/.config directory, then ajust it to your needs.

_:

[redshift]
# Set the day and night screen temperatures
temp-day=5800
temp-night=4800

# Enable/Disable a smooth transition between day and night
# 0 will cause a direct change from day to night screen temperature.
# 1 will gradually increase or decrease the screen temperature
transition=1

# Set the screen brightness. Default is 1.0
#brightness=0.9
# It is also possible to use different settings for day and night since version 1.8.
#brightness-day=0.7
#brightness-night=0.4
# Set the screen gamma (for all colors, or each color channel individually)
gamma=0.9
#gamma=0.8:0.7:0.8

# Set the location-provider: 'geoclue', 'gnome-clock', 'manual'
# type 'redshift -l list' to see possible values
# The location provider settings are in a different section.
location-provider=manual

# Set the adjustment-method: 'randr', 'vidmode'
# type 'redshift -m list' to see all possible values
# 'randr' is the preferred method, 'vidmode' is an older API
# but works in some cases when 'randr' does not.
# The adjustment method settings are in a different section.
adjustment-method=randr

# Configuration of the location-provider:
# type 'redshift -l PROVIDER:help' to see the settings
# e.g. 'redshift -l manual:help'
[manual]
lat=43
lon=1

# Configuration of the adjustment-method
# type 'redshift -m METHOD:help' to see the settings
# ex: 'redshift -m randr:help'
# In this example, randr is configured to adjust screen 1.
# Note that the numbering starts from 0, so this is actually the second screen.
[randr]
screen=0

Of course, you can play with the temp-day and temp-night values the gamma, and you will have to provide the correct values for “lat” and “lon” according to your geographical position.