Our printers use CMYK (Cyan, Magenta, Yellow & Black combinations) inks and will not always translate PMS (Pantone Matching System®) colors accurately or perfectly.  
This service is strongly recommended for color-critical projects with referenced PMS colors. 
We do not print actual Pantone colors but for this service, we try our best to match as closely as possible with CMYK. 

A hard copy proof can be very helpful in deciding which material/saturation level to choose before placing your order or to simply see a preview before printing your graphic(s).