It may be difficult for you to imagine at first, but these four colors will mix together to create the image above. Where the Web world applies hexadecimal codes to its colors, the print world applies PMS, or Pantone Matching System numbers, to its spot colors. Coated and uncoated colors are close, but not always identical. Stock choices can range in weight, as well. A text weight is generally used for letterhead, while cover stock is used for business cards.
Envelopes come in different shapes and sizes.
Ask the print house if you can see samples of each. If the cost cannot be rationalized and you think specifying spot colors will be an isolated incident, visit the print house and choose the PMS numbers you'd like to use from their guide. Every good printer has one and they'll need these numbers from you before they can print the job. If you're doing long-distance printing and know of a friendly local print house, you could also drop in to have a quick look at their PMS guide. Bring a printed version that matches your colors as closely as possible so you can specify your PMS colors from an established reference point.