How do I choose the best color profile for a LG external monitor?

What you want to do is adjust the color profile on your Macbook Pro to take advantage of the LG 22MP55. So you would go to System Preferences -> Displays and then choose Color and play around with the selected Display profile: to find one that suits your needs best.

Or you can take it a step further by calibrating your own profile. Which can be tricky, but does work if you spend the time and effort. In that same preference area choose Calibrate… to begin the process of adjusting your screen settings to best suit your needs.

But past any of that, you might find it worth it to invest in a calibration device such as a Spyder calibrator. I use the Spyder3EXPRESS and it works great. The newest version is the Spyder4EXPRESS which should be just as good if not better. It costs anywhere from $60 to $99 and is well worth the investment. The color on your display will be perfectly calibrated with little hassle or headache.