Solar heating of a cometary surface provides the energy necessary to sustain gaseous activity, through which dust is removed(1,2). In this dynamical environment, both the coma(3,4) and the nucleus(5,6) evolve during the orbit, changing their physical and compositional properties. The environment around an active nucleus is populated by dust grains with complex and variegated shapes(7), lifted and diffused by gases freed from the sublimation of surface ices(8,9). The visible colour of dust particles is highly variable: carbonaceous organic material-rich grains(10) appear red while magnesium silicate-rich(11,12) and water-ice-rich(13,14) grains appear blue, with some dependence on grain size distribution, viewing geometry, activity level and comet family type. We know that local colour changes are associated with grain size variations, such as in the bluer jets made of submicrometre grains on comet Hale-Bopp(15) or in the fragmented grains in the coma(16) of C/1999 S4 (LINEAR). Apart from grain size, composition also influences the coma'
