randcolor outputs an RGB hex code with random hue and constant value. I just use this so every new terminal's background color is random but dark, so as to ensure that light text is always readable and easy to see.
$ randcolor 0.1
#190018
How it's useful:
$ awk '/randcolor|urxvt/' /etc/i3/config
set $urxvt exec urxvt -bg $(randcolor .05)
bindsym $mod+Return $urxvt
bindsym $mod+F1 $urxvt