2025ApJ...991..193L

The James Webb Space Telescope (JWST) has provided critical mid-infrared data for cold brown dwarfs. It has also provided low-resolution near-infrared spectra, and for faint sources these are the first spectra at these wavelengths. We use these data and other literature sources to synthesize near-infrared photometry on the Mauna Kea Observatories (MKO) system for 19 T and Y dwarfs, on the Euclid system for 44 T and Y dwarfs, and on the Roman systems for 48 T and Y dwarfs. We also synthesize Euclid I_E magnitudes for 15 T and Y dwarfs. Using the S. A. Beiler et al. () observational effective temperatures (T_eff), together with ATMO 2020++ model colors, we show that the absolute 4.6 μm magnitude can be used as a proxy for T_eff. We present a polynomial fit to the M_W2:T_eff relationship for cool dwarfs with 250 ≲ T_eff K ≲ 1000. We select five Y dwarfs with 275 ≲ T_eff K ≲ 400 that have a range in near- to mid-infrared colors. Comparison of the JWST spectral energy distribution to ATMO 2020++ models indicate that Y dwarfs that are bluer in J - W2 are lower gravity or more metal rich than their redder counterparts, with stronger CO and CO₂ absorption at 4.2 ≲ λμm ≲ 4.9. The near-infrared color diagrams show significant scatter, with complex dependencies on T_eff, metallicity, and gravity. In order to disentangle these effects, opacity sources for cool model atmospheres need to be more complete at λ < 1 μm.