We study the converse problem of retrograde axonal transport, where neurons in the brain inform the photoreceptor neurons, via their synaptic connections, of the spectral sensitivity to which to tune their light reception. A subset of photoreceptor neurons in the fly retina is dedicated to the reception of the spectral information for color vision. These neurons choose to express different light-sensitive pigments (Rhodopsins). We have found that the neurons in the brain, to which the photoreceptors communicate their light reception, transmit a retrograde signal that determines which Rhodopsin genes the photoreceptors express. By coordinating Rhodopsin expression in this way, the neurons in the brain determine the color of light that they are detecting, and therefore can properly encode this color input in the visual field.