I bought two Asus PB238Q for my dual monitor set up. I have one hooked up via the DVI and one through the Displayport. The monitor that is hooked up by the Displayport seems to have washed out colors and definitely does not match the DVI monitor. I have the exact same settings, but it seems washed out and no matter what I do I cannot get the colors to match.
Both being digital interfaces, I do not believe it is the cause. Most likely is that you have some form of image processing enable either on Windows, your display driver, or the monitor OSD. If not that, there could be slight variance in the monitors factory calibration. You would need to recalibrate the monitors to solve this, which require a monitor calibrator. Some shops might be able to do this for you.