If you could tune up a Synthesizer into the electromagnetic spectrum of visible light (radiation with a wavelength between 380 nm and 760 nm) then you would get colour out of the speakers when playing the keyboard.
Now, that may be to exaggerate a little, but you can play colors with sound by dividing the electromagnetic frequencies mathematically, down to suite the audio spectrum.
Echad utilises PSL (Phase Shift Lock) in the TRT song versions. PSL is a frequency enhancement technique to make the music even more effective.
More about the concept: www.echad.se
Headphones is recommended.