That depends on what keyboard you have now. If you have a 2008 Mac Pro and an Apple thin aluminum keyboard, then your only choice is what logickeyboard calls a skin. It is one piece, rubbery, and fits completely over the keyboard
If you have a 2007 Mac Pro, then you have a choice: either a complete keyboard with the color coded keys, or just the color coded keys that you install on your existing keyboard. This package includes a key puller that you use to remove the current white keys.
For my FCP clients, I spec the keycaps for an older Apple Pro Keyboard, and the skin for the 2008 aluminum KB. I would only choose the complete keyboard is the client wanted to have a backup keyboard in case of failure, and I've never had to do that.