Agar plate with bacteria colonies  transfected with various fluorescent proteins.  The "Reflected Light" image was taken using room light.  The observed colors are due to the absorption spectra of the fluorescent proteins.  For example, a red colony means that the protein absorbs the blue and green portion of the spectra while reflecting the red portion.  The "Fluorescent Image"  was taken in a dark room, while the agar plate was illuminated with ultraviolet (UV) light.  The observed colors are due to the fluorescence of the various  proteins.

NOTE: You will have to enable ActiveX Control for the animation to display properly.