1. An ordered dithering method for processing a starting image having a first chromatic resolution and formed by N×M pixels, for obtaining an output image having a second chromatic resolution, said second resolution being lower than said first chromatic resolution, said starting image being suitable for being decomposed in three color channels, respectively R (red), B (blue) and G (green), and a brightness channel (A), said method comprising the steps of: predisposing a first and a second dither matrix, each of said matrices comprising a multiplicity of threshold values ordered in j rows and j columns, where j is smaller than both N and M, repeatedly applying the first dither matrix to the R, B and A channels of the starting image and the second dither matrix to the G channel of the starting image for comparing the value of each of the pixels that compose said channels with a selected respective threshold value belonging to the respective dither matrix; wherein said first and second dither matrices comprise the same threshold values, said threshold values having a different spatial resolution within said two matrices.