In this case, PMS, refers to the Pantone Matching System and not your irritable female friends. There are generally two types of color systems used by printers. The system used most, called process colors or CMYK, is a combination of four standard inks: Cyan (blue), Magenta (Red), Yellow and Black. These four are used in varying percentages to create all the colors you would see in a Pottery Barn catalogue, a sailing calendar or the piles of colorful junk mail that are delivered to your front door. The printer sends a blank piece of paper through the press and four separate times the paper receives ink from four different printing plates (one plate each for Cyan, Magenta, Yellow and Black).

The other color system, the PMS or spot system, is used mostly for two color projects. Spot colors are mixed before they are used on press. So the particular shade of mint green you want is not created by pressing a little yellow, a little blue and a little black separately on the paper as it goes through the press. Instead the mint green is premixed and laid down with one single touch of the paper. Spot colors are fantastically predictable so that if you choose a PMS swatch with your neighborhood printer, you are pretty much guaranteed to have the exact shade of mint in your hand when the project is finished.

The Pros of Spot Color Printing

• As already mentioned, it's very predictable. The inks are mixed in exactly the same proportions every time. Whereas with CMYK printing, there are a lot more variables. The ink is transfered from rollers to plates to blankets to paper and every step can vary in pressure and absorption, etc. There is a little more guessing involved in preparing files for printing on a four color press.
• Spot colors are often cheaper because the paper only needs one or two hits
• PMS colors can be brighter or sharper than CMYK colors. Sometimes PMS colors are even added to CMYK projects when a particular color can't be achieved by process colors alone.

The Cons

• You can't really use PMS inks if your your project has more than two colors. Well, you can, but it gets complicated.
• The biggest drawback to using spot colors is that there is no way to get an exact match between spot colors and CMYK colors. Which means that if you print business cards in two color now but you want to use the same branding on a brochure with photographs later, your mint greens will look slightly different. If you're someone who feels very picky about your colors, it's best to print all of your materials in CMYK from the beginning. Even your two color business cards. The amount of difference between PMS and CMYK depends on the color. With muted subtle colors (steel blue, forest green, dusky rose) you won't see much difference at all. With a brilliant blue or peach, you're going to see a pretty significant difference.

Your Options

When choosing whether or not to use spot colors you have three options:

1. You can print simple projects like letterhead and envelopes with PMS colors and live with the difference in color later when you print brochures or any sort of multi-colored pieces.
2. You can print your pieces with CMYK colors now, pay a little more but know that all your pieces will be really consistent. 
3. You can print with PMS colors now and later add PMS colors to your CMYK pieces. It's more costly to run 5 and 6 color pieces on press and not all printers can do it. But bigger companies who feel very strongly about consistency in branding do it all the time. For smaller businesses and non profits, it's not the best option. In truth, most people aren't going to hold your brochure and your business card up side by side and shake their heads at the lack of quality control.

If you are concerned about the possible difference between your two color projects and your CMYK projects, you can ask your printer to show you a Pantone Solid to Process guide. It's a swatch book with their whole color library shown both in spot color form and process form. That way you'll know what to expect.