a | several spectrally distinct fluorescent proteins (XFPs) are now available, including ones that emit in red (RFP), green (YFP) and blue (CFP) frequencies. b | The combinatorial expression of red, green and blue XFPs at various levels is sufficient to encode a colour space analogous to the one that is generated by an RGB video monitor. c | An example showing how ten distinct colours can be generated by expressing a trimeric combination of three different XFPs. In Brainbow mice, this outcome would result if three copies of a trichromatic transgene (illustrated at the top of the panel; see Box 1 for details) each recombined independently (Box 1). Triangles represent lox sites (see Box 1 for details). CFP, cyan fluorescent protein; P, promoter; RFP, red fluorescent protein; YFP, yellow fluorescent protein.