You're working with 3-4 variables here. This will make your head hurt.
The first variable is probably obvious, the stains/coatings themselves. Being a bit of a DIY nerd myself, I know this is already going to vary over time, by application, and by surface. But this is probably already assumed, and your client will choose whatever ideal application they wish. But that doesn't mean that when they open the can and make a new swatch that it'll still match your poster--and neither should you guarantee that.
The second variable is the input method for these stains into a digital image file. Are you taking a photo? Are you scanning a photo? Are you scanning the stain itself? These will all produce different results, or possibly a myriad of results within each method. They also expose you to the first set of color expression variables such as color profiles, lighting, and camera/scanner "color enhancement" effects.
The third variable is converting that image into the print color space. I hope you already understand this, but in case you don't, I'll explain briefly. Screen displays store red, green, and blue (RGB!) colors that combine make white, or when absent, make black. This is an additive color model (left, below), for when there is a light source present behind the color (or displaying the color.. but I digress). In print, it's inks on paper, which necessitates that adding colors make a color darker and/or more complex, and that is a subtractive color model (right, below)... see the cyan, magenta, and yellow?
It's just never going to be the same on-screen. If you can get pseudo-true-to-life color via a color system and print system, then you should completely ignore what it looks like on screen. If you need some sort of on-screen presence, like a website or interactive kiosk or somesuch, then you're going to have to keep a completely separate base of these scanned/photographed images for screen work vs print work.
Anyway, hopefully this is all very elementary to you, but if not, consider this your warning that you definitely are in over your head and you should take at least a primer class or two before you take on another such project.
The fourth is of course the printer, and since we don't really know how you're running this (print shop/their setup, color laser, color inkjet, etc), I can't really say more. But if you are having it professionally printed, and you don't know how to send them the right stuff, suffice it to say that they will gladly print it out the way it was provided to them regardless of if it was what you wanted.
