Surface brightness-color relations in V − K. Black points represent the 526 RGB stars used in this study to fit the SBCR, with their surface brightness estimates derived from asteroseismic radii and Gaia distances. Open blue squares denote the RGB and RCG samples from the literature (Pietrzyński et al. 2019; Salsi et al. 2020, 2021) used to calibrate the SBCR, with their surface brightness estimates based on angular diameters measured via infrared interferometry. The solid red line illustrates the SBCR fitted in this work, with dashed lines indicating the associated uncertainties (1σ). The dashed magenta line, the solid sky blue line, the dotted green line, and the dashed orange line represent SBCRs proposed by Pietrzyński et al. (2019), Nardetto et al. (2020), Salsi et al. (2021), and Graczyk et al. (2021), respectively. The lower panel displays the residuals of the fit.