I have a macbookpro (late 2013 retina model - 11,1) running xubuntu 13.10. I have the main laptop monitor and two Asus VS238H monitors. One connected by DVI and another by HDMI.
Currently, one external monitor has very washed out colors, the blacks almost look gray.
I have used different cables and can make the washed out colors appear on the alternate external monitor by switching the HDMI cable from the second monitor to the first.
Therefore, it seems to me that this color difference is something caused by the software and not the cables or monitors themselves. I've looked at xrandr settings (gamma and brightness) but that doesn't get the monitors to look the same.