The eye contains only three kinds of colour detecting cone cells: red, green and blue. Colours that fall between them appear perceptually brighter. The “height/lightness” of spectral colours is also inverted in comparison to the pitch spectrum: higher frequency colours are perceived as darker and heavier. The picture below shows the response curves of RGB cones mapped to the colour spectrum and illustrates how the "lighter" anomalies occur at the mid-points between them.
Lyrebird at present allows for the following mappings of timbral brightness to hue. The spectra below depict a test tone of increasing brightness, noisiness and bark scale depicted by a variety of mappings.
In addition to functioning as a score player for filed recordings, Lyrebird my have some application as a tool for feature analysis of electroacoustic works. Below are some sample mappings of Pierre Schaeffer's Etude au Chemin de Fer, in which contours and timbral shifts are readily recognisable. The scroll-rate is slowed to 200ms/px so that the entire work can be seen in a single pane. Interestingly although the mappings "YORVIB" and "YGBIV" appear to give the best illusion of gradation between light and dark they don't necessarily provide the best results for this particular bit of sound.