SBCRs in the literature that are consistent with an application to Cepheids. Top: SBCR in the literature plotted over the (V − K)₀ validity domain. The gray zone correspond to the rms of K04 SBCR, i.e., 0.015 mag. The rms of other SBCRs (in mag) is indicated in the legend. Bottom: SBCRs compared to K04 in the (V − K)₀ range corresponding to η Aql pulsation. SBCRs that are partly or not at all consistent with the (V − K)₀ range of η Aql (see Table 5) are not indicated in this second panel.