Updates:This question describes similar symptoms Ubuntu - Xrandr doesn't change my resolution, but it seems like the underlying cause is different as there are no changes to my Xorg-log files in the last 6 months.Another similar question without answer xrandr --pos doesn't do anything My external screen is too bright, even on the lowest brightness setting. Too dampen the brightness, I am trying to use xrandr --output HDMI2 --brightness 0.5 as detailed in this post How do dim screen, even if artifically, below the minimum?.However, while this command does lower the brightness, the screen switches back to the brighter level in an instant. Basically, the screen blinks once in a darker brightness. I tried with my primary laptop screen, xrandr --output eDP1 --brightness 0.5, with the same outcome. I also tried changing the gamma, xrandr --output eDP1 --gamma 1.0:1.0:0.5, again with the same transient result, this time a one second flash in a yellow tint.How can I make xrandr modifications persist until I specifically change it? If it persists through reboot or not isn't a big deal.My xrandr is version 1.5.0 and I am on Antergos Linux (an Arch derivative) with Gnome 3.20.2 and kernel 4.6.2-1. 
How can I make xrandr modifications persist for more than 1 second?
xrandr;dual monitor;brightness