A chromatic diffusing layer (510) comprises a plurality of nanoparticles (37) embedded in a matrix (39), for Rayleigh-like scattering with an average size d in the range 10 nm≤d≤240 nm, and a ratio between the blue and red scattering optical densities Log [R(450 nm)]/Log [R(630 nm)] of said chromatic reflective unit falls in the range 5≥γ≥2.5, where R(λ) is the monochromatic normalized specular reflectance of the chromatic reflective unit, which is the ratio between the specular reflectance of the chromatic reflective unit and the specular reflectance of a reference sample identical to the chromatic reflective unit except for the fact that the chromatic diffusing layer does not contain nanoparticles with the size d in the range 10 nm≤d≤240 nm and for the direction normal to the reflective layer (508) of the chromatic reflective unit (506), the monochromatic normalized specular reflectance R(λ) of the chromatic reflective unit at a wavelength of 450 nm is in the range from about 0.0025 to about 0.15, such as defined by the equations 0.0025≤R(450 nm)≤0.15, 0.0025≤R(450 nm)≤0.05, 0.0025≤R(450 nm)≤0.04.