A molecule is fluorescent if when you shine a specific wavelength of light on it, it will respond by sending out light of another wavelength. For instance, the Green Fluorescent Protein GFP reacts to blue light and emit green light as a consequence.
Characteristics of fluorescent proteins
Most coloured molecules found in nature (animals and plants) are the result of complex, multi-steps reactions. For instance, the red colour of roses needs at least five different proteins to be produced from a non-coloured molecule (note however that the colours of flowers and many other things in nature are not produced by the process of fluorescence). Fluorescent proteins are different because their production does not need intermediate steps or the intervention of other molecules: they can act on their own structure to produce a fluorescent colour. All a scientist needs to do to get that colour into a fish is to introduce the coding sequence of the gene into the genetic code of the fish; the protein that is produced will take care of everything else.
Origin of fluorescent proteins
GFP was originally identified and isolated from a jellyfish Aequorea victoria in 1962 by Osamu Shimomura. In the jellyfish it constitutes a component of a bioluminescent organ that lights up in the dark. The sequence of the gene was determined in 1992. When people realised its useful properties for research, new expeditions to find similar proteins among marine life were launched.
Genetic modification of the original and newly found proteins was used to improve their brightness, stability and colour. As a result of these efforts we now have a palette of colours covering the full spectrum of visible light.
Advantage of fluorescent proteins
Compared to non-fluorescent colours, fluorescent proteins are very bright. So bright that we are able to detect tiny amounts of them under a microscope. This is particularly useful to detect the localisation of almost any organ and sub-cellular component.
Though other methods exist to access the same information, the use of fluorescent proteins is not invasive and thus allow scientists to access these information in a living organism.