This work for flute, clarinet, piano, violin and cello, written to a commission from the Goethe-Institut in Paris, is one of the composer's most frequently performed and the earliest realization of live electronics. For the first time in his production of the 1970s, Murail inaugurates the technique of ring modulation in an electroacoustic situation, optional for public performance. The work's form can be deduced from the 13 generating intervals played by the flute and clarinet. The modulator calculates the additional and differential frequencies and enriches the sounds played by the strings. The objective of the modulation is to create shadows in certain transitional passages between sections, often announcing or doubling the sounds played by the strings in those same places.
In Ethers (1977), Murail had already experimented with the wealth of complex sounds produced by simultaneously singing into and playing the flute, the resultant sounds then being transferred to the strings. In Treize couleurs, the composer develops this procedure, combined with a concept of premonition or prolongation of complex sounds. The model of the ring modulator allows the composer to organize a sound trajectory whose frequencies generate themselves in a chain reaction.
After an introductory sequence around the frequency E, the polar but fluctuating note, the entrance of the piano marks the first colour on E flat. Murail offers us variations of light with relations of one colour to the next by harmonic drift. He explains his process as follows: "Starting from medium brightness in the first sections, the timbre progresses towards a reverberating light marked by a very tight interval in the upper range (6th colour), to arrive at a dusk that, at the very end, sounds like a knell." With his series of paintings of the same subject at different times of the day (haystacks, poplars, cathedrals), Claude Monet attempted to measure the degrees of light and tints. Murail, in an analogous process, through the repetition of the same musical object within each section, transports the listener from one round colour to another through thirteen successive shades, going from dim to brilliant, pure to saturated, from incandescent bursts to the progressive removal of light intensity.