Lost in Space - Danger Will Robinson, Danger!

Colour Space Explained
  It looked blue on my computer monitor, but when my print came back from the printer it was purple... what happened? It's all about colour space.
 Computer monitors transmit colour as RGB (red, green, blue) light. Although all colours of the visible spectrum can be produced by merging red, green and blue light, monitors are capable of displaying only a limited gamut of the visible spectrum.
 Monitors transmit light, inked paper absorbs or reflects specific wavelengths. Cyan, magenta and yellow pigments serve as filters, subtracting varying degrees of red, green and blue from white light to produce a selective gamut of spectral colours.
 Like monitors, printing inks also produce a colour gamut that is only a subset of the visible spectrum, however the range is not the same for both. Therefore, the same art displayed on a computer monitor may not match to that which is printed. Print processes normally use CMYK (cyan, magenta, yellow, black) inks requiring digital art to be converted from RGB to CMYK colour for print.

                  
                                               Red, Green, Blue - Additive colours

                
                                        Cyan, Magenta, Yellow - Subtractive colours

 Printers may prefer your files in the CMYK (Cyan/Magenta/Yellow/Black) mode, as this is the colour mode for the printing process. If an RGB (Red/Green/Blue) file is submitted, it must be converted to CMYK for print.

                                 
                                            Visible colour spectrum with print gamut

 It can sometimes be difficult to visualize the reason for colour shift in colour space conversion. The best way to see the differences between the CMYK and RGB colour spaces is to look at a colour gamut comparison chart. The chart above plots the visible colour spectrum as the larger area, and within this is a plot of the CMYK and RGB colours. Some areas of the RGB colour space are outside that of the CMYK space. It is these colours that are affected by a conversion from RGB to CMYK. Some digital printers have expanded to additional shades of cmyk expanding the gamut capabilities of traditional print, but for the most part expect some colours to be "out of range" to conventional printers.
 For all the "Lost in Space" fans out there...