Match second monitor settings to first
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.