Schematic showing why the trajectory in the colour-colour plane flattens, as shown by the break in the dashed black lines in the left-hand panel, rather than following the standard JP model (dashed grey lines). Lines of sight initially intersect homogeneous CR electron populations (top right panel), with the same steep spectral curvature. Due to turbulence, later lines of sight intersect inhomogeneous populations (bottom right panel), made up of superpositions of cooled and fresher spectra (dashed purple and lilac lines, respectively). The resultant weaker spectral curvature correspondingly flattens the trajectory in the colour-colour plane. The effect is exaggerated here for explanatory purposes.