When it comes to printing packaging, labels and stationery, people often specify the colours they want in terms of their pantone colours. While this is a very useful way of letting the printer know what shades of colour you want, it is a mistake to think that he will be able to give you exactly the same colour as you see in the Pantone book or on the website.
The Pantone number refers to a recipe. It tells the printer that if he mixes a certain number of grams of ink colour A with a given quantity of colour B (there is often a C and a D also) he will end up with the required colour ink. If he sticks to these proportions, he will always get the same colour. While this is a great starting point, it is just a starting point... because the ink is not meant to stay in the tin, it has to be printed on to paper.
The substrate (paper or plastic) has a huge baring on the final colour that is achieved. The inks are not fully opaque so the colour of the paper can be seen through the ink and so will effect the final colour as you see it. Also, some papers will absorb the ink and so 'kill' its vibrancy making it a duller shade than it appeared in the tin.
Another factor to be aware of is mechanical. While printing presses are getting better and more consistent all the time, you can often get different results from different presses. Variations can occur in the weight of ink (the thickness of the layer) laid down and, even on the same press there can be variations within a run and variations between runs due to purely mechanical issues.
All that being said, it is quite possible to get what you want if you plan it properly. I'll go through all that in the next article.