Overview of the spectral models used in this work (left) and corresponding radio color–color corves (right) to highlight differences in the spectral shapes. All spectral models have α_inj = 0.6. Solid grey line and filled grey circle (in the color–color diagram): power-law spectrum without any ageing, solid black line: KP, dotted line: CI, dash-dotted line: KGJP, dashed line: JP. Note that a power-law spectrum will translate in a single point on a color–color diagram. The α_{low−frequency} = α_{high−frequency} line divides concave and convex spectral shapes between the frequencies where the spectral index is computed.