You need a calibrating device... I recommend the
X-Rite i1 Display Pro
It comes with it's own software, and will display a series of colour swatches and grey scales on the screen of a known value. The colorimeter will then measure the actual colours your display is providing, and the software will make adjustments to bring what's being displayed in line with what the values of the swatches should be. It then writes these differences into a monitor or .ICM profile that your Mac will load into your video card's LUT (look up table) each time your Mac boots. This way, what is displayed will always be accurate.
Certain high end screens will allow "Hardware profiling" which is essentially the same, but the .ICM profile is loaded into the screen's LUT instead, which usually operates at a higher bit depth for greater accuracy and tonal gradation. Your Mac monitor can only be software profiled, but don't worry too much about that.
If your Mac Pro has a Quadro or Fire card, there will be no advantage to hardware profiling however, as these cards have a 10bit LUT.
If your Mac screen is not too old, it should still calibrate well, and seeing as it was bought in 2014, this should be the case.
If and when you get a calibrator, PM me and I can advise further.
[edit]
As you are doing your own printing, there may be a case for your getting the
X-Rite Color Munki Photo, as this can also calibrate your printer ink and media. Having a calibrated screen is no guarantee of print accuracy alone so despite the i! Display Pro being superb, in actuality, if you want to be completely independent of third party printing, the Color Munki Photo may be an ideal one stop solution to calibrate your entire workflow.