We Strive for the Best Possible Matches, however...
COLOR IS RELATIVE
From a can of specially mixed Pantone ink on an offset press... to how a CMYK laser digital press using powdery toner will interpret that same Pantone color... to how the large format's micro piezo wet ink jet system will print that same color... to how dye sub, or screen printing, CAD-cut adhesive vinyls, to spray or brush on paints for signs, or embroidery threads will strive to come close...
COLOR MATCHING CAN BE CHALLENGING EVEN WHEN WE FOLLOW ALL THE RIGHT RULES
Then change the paper from matt to glossy, then to fabric or a coffee mug...
Another thing to keep in mind is that your computer monitor may not be giving a true picture (especially if it is not of a calibrated professional level -- and especially if it is a backlit LCD style - which makes everything BRIGHTER then it will look once printed), your personal or office copier or inkjet printer will have it's own system of interpreting and printing colors that will differ from our equipments rendering of the image.
The time of year, heat, cold, humidity and barometric pressure can also affect the quality of the final prints and cause a shift in hues ~ so even when we are using the same paper on the same machine, implementing all the controls available to us ~ things may not be a perfect match.
Just as with Pantone logo colors ~ photographs and artist renderings can shift in color as well.
CMYK LASER DIGITAL HARDCOPY PROOFS: When we are going to be running your order on our Xerox or Canon equipment, we prefer that you see a printed sample, off that equipment, on the actual paper we will run the job on (and not an emailed/screen proof).
If your job will be run on an offset press (or even our Large Format Photo Printer) it can be costly to provide actual samples for your proof... unless you want to make that added investment, it comes down to a matter of trust ~ that you will be able to let us make the judgement call once the more expensive equipment is rendering your order.