I use `xflux` from f.lux on my OpenBox desktop, it's a simple command line tool for X Window System with multiple monitor support. You can download it here.
Just download the archive, extract it and move the `xflux` binary file to a directory registered in your `$PATH`. Then add to you `~/.config/openbox/autostart` file the line
xflux -l latitude -g longitude
Depending on your location, the program will automatically adjust the light. You can also change the color with the `-k` flag.