Redshift is a free utility program for Linux that allows you to adjust the computer screen’s color temperature.

You’ll want to use the software to reduce eye strain, especially at night. The bluish color of the computer display is hard on your eyes.
Redshift colors the screen in a warmer, reddish color.

Minimal Installation of RedShift and Setup (Arch Linux)

Installation

Using the Arch package manager with yay:

yay -S redshift-minimal xorg-xbacklight

Setup

Go to http://www.geonames.org/ and search for your location. Write down latitude and longitude.

Create ~/.config/redshift/redshift.conf with the following content:

; Global settings for redshift
[redshift]
; Set the day and night screen temperatures
temp-day=5700
temp-night=3500

; Disable the smooth fade between temperatures when Redshift starts and stops.
; 0 will cause an immediate change between screen temperatures.
; 1 will gradually apply the new screen temperature over a couple of seconds.
fade=1

; Solar elevation thresholds.
; By default, Redshift will use the current elevation of the sun to determine
; whether it is daytime, night or in transition (dawn/dusk). When the sun is
; above the degrees specified with elevation-high it is considered daytime and
; below elevation-low it is considered night.
;elevation-high=3
;elevation-low=-6

; Custom dawn/dusk intervals.
; Instead of using the solar elevation, the time intervals of dawn and dusk
; can be specified manually. The times must be specified as HH:MM in 24-hour
; format.
;dawn-time=6:00-7:45
;dusk-time=18:35-20:15

; 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=0.8
;gamma=0.8:0.7:0.8
; This can also be set individually for day and night since
; version 1.10.
;gamma-day=0.8:0.7:0.8
;gamma-night=0.6

; Set the location-provider: 'geoclue2', '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.
; ex: 'redshift -l manual:help'
; Keep in mind that longitudes west of Greenwich (e.g. the Americas)
; are negative numbers.
[manual]
lat=<insert your city's latitude>
lon=<insert your city's longitude>

; 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 only screen 0.
; Note that the numbering starts from 0, so this is actually the first screen.
; If this option is not specified, Redshift will try to adjust _all_ screens.
[randr]
screen=0

Adjust the lat and lon entries. Provide longitudes in the western hemisphere (e.g., the Americas) as a negative number, see FAQ.

Create a hook script to use real screen brightness. Create the file ~/.config/redshift/hooks/brightness.sh:

#!/bin/sh

# Set brightness via xbrightness when redshift status changes

# Set brightness values for each status.
# Range from 1 to 100 is valid
brightness_day=100
brightness_transition=50
brightness_night=10
# Set fade time for changes to one minute
fade_time=60000

if [ "$1" = period-changed ]; then
	case $3 in
		night)
			xbacklight -set $brightness_night -time $fade_time
			;;
		transition)
			xbacklight -set $brightness_transition -time $fade_time
			;;
		daytime)
			xbacklight -set $brightness_day -time $fade_time
			;;
	esac
fi

Make the script executable:

chmod +x ~/.config/redshift/hooks/brightness.sh

Enable the systemd user script:

systemctl --user enable redshift.service --now

Voilà!

Further Reading