What do you mean the colors are "terrible"? Are they tinted, washed out, too dark, too bright...?
Also I can't find the XL2311 listed anywhere - do you mean the XL2411Z? If so, that is a TN monitor, which means that it will have not-so-great viewing angles (if you look at it from any angle other than head-on, the colors will shift). Additionally, it may be a 6-bit panel with FRC to emulate 8-bit color depth; if so, depending on the quality of the FRC implementation color accuracy may be off.
Finally, the monitor could just have horrible color calibration out of the box, and this is the reason why it looks off to you. I'm not sure how extensive the OSD controls are for this monitor, but you may be able to partly calibrate the monitor yourself through that.
If you want more thorough calibration and assuming you do not have the proper hardware/software to do it yourself, you can grab an ICC profile from here:
http://www.tftcentral.co.uk/articles/icc_profiles.htm#the_database
The XL2411Z isn't listed but you should be able to use the ones for the XL2420, since they both use the same panel. Download the icc profile and have windows apply it through color management settings.