Match second monitor settings to first I'm new to using the Mac. I'm a photographer, use a MacBook and recently purchased a Mac monitor (discontinued, bought it second-hand) to connect to my MacBook as a second monitor for my photo editing, lightroom, photoshop etc. 

On this site I learned how to adjust brightness on the second monitor (Control + F1, F2) but I need to know how to match all monitor settings in the two monitors, such as contrast, colour saturation and so on.

At the moment my photos look slightly different on each screen. With my MacBook setting being the the one I wish to duplicate on the second monitor.
Apple includes manual color calibration and correction:


https://support.apple.com/kb/PH19044


You may not be able to match the external to the internal if the quality of the LCD or age/manufacturing of the backlights vary. If you need the best displays for color look to Enzo or NEC. As they age, you may have to adjust things, so consider hardware to calibrate if you have to certify your images or there is $ on the line when your images aren't perfect. Most people get by with Apple's tools and a correction every six months or so.