The NIR color–magnitude diagrams of NGC 6441. Left: the color–magnitude diagram is not corrected for extinction and the reddening vector is shown at the top of the panel. The representative error bars are ±5σ in magnitude and color. Top right: close-up of the RRL stars on the horizontal branch of the extinction-corrected (J − K_s), J color–magnitude diagram. The horizontal branch stellar evolutionary models (Pietrinferni et al. 2006) for canonical helium (Y = 0.256) and enhanced helium content (Y = 0.350) are also shown. A distance modulus of 15.52 mag (Baumgardt & Vasiliev 2021) is adopted. Bottom right: close-up of the RRL stars in the extinction-corrected (J − K_s), K_s color–magnitude diagram. The dotted and dashed lines (with ±1σ errors due to distance and reddening uncertainties) represent predicted magnitudes and colors for RRL stars in our sample using the theoretical PLZ relations of Marconi & Minniti (2018) and Marconi et al. (2015) with and without a helium term, respectively. The dash-dotted blue and solid red lines represent the predicted blue edge of the first-overtone mode and the red edge of the fundamental mode, respectively.