Simple Colour Organ is a simple OpenGL plugin based on the 'colour organ' ideas of people like Father Louis Bertrand Castel and A. Wallace Rimington, where musical notes are seen as corresponding to specific colours.
The plugin is strictly a visual instrument – it does not output any sound. Instead colour is displayed when you hit a key. There are 7 colour palettes included – 6 of which are based on the palettes by the aforementioned Father Castel and Rimington, as well as Alexander Scriabin (all obtained from here), while the seventh is a pastel 'scale' of ndc Plugs own devising. You can add your own palettes. The palettes can be changed while the plugin is running by using the left and right mouse buttons to scroll through them.
The plugin also has 2 'hidden' parameters, accessible via automation, which control the attack and delay values of the colour envelope.
A good primer on colour organs and the idea of 'colour music' can be found here.