I'm working on setting up a binding to set brightness/color temperature in a way similar to the program redshift does with randr. http://jonls.dk/redshift/
Redshift and f.lux both work fine with xmonad.
Just a first look, I see redshift calling xcb_randr_set_crtc_gamma from #include <xcb/randr.h>, and I'm wondering how to do the equivalent in xmonad.
If for some strange reason you must do it with xmonad (this is not a particularly good idea, especially considering that the Xlib API appears to require callbacks), you want the XRandR bindings for Xlib. I don't see any bindings on Hackage, and neither the Haskell X11 package nor xmonad provides them, so expect to learn how to use hsc2hs. "man 3 Xrandr" for the Xlib API, although the manpage I have locally is rather sparse so you may need to look for more information elsewhere.
Alternately, you can "spawn" the xrandr shell command; see the -gamma option.