Posterior distribution of the chromatic index (blue violins) and their 99.7% credible intervals (errorbars) obtained for the pulsars that favor the presence of CN₄ in their noise model. Here we use the favored noise model and replace the CN₄ noise component by a Gaussian process power-law PSD signal with the same number of frequency bins, but a free chromatic index, with a prior defined as 𝒰(0, 10). The black horizontal line displays the fixed value for the case of CN₄.