Brainbow is the process by which individual neurons in the brain can be distinguished from neighboring neurons using fluorescent proteins. By randomly expressing different ratios of red, green, and blue derivatives of green fluorescent protein individual neurons, it is possible to flag each neuron with a distinctive color.
This process has been a major contribution to the field of connectomics, or the study of neural connections in the brain. The technique was originally developed in the spring of 2007 by a team led by Jeff W. Lichtman and Joshua R. Sanes, both professors of Molecular & Cellular Biology at Harvard University. (Source)