Evolution of the optical and NIR spectral slopes of 3I/ATLAS as it approaches perihelion. The dashed vertical line indicates heliocentric distance where 3I/ATLAS crossed perihelion. While the optical slope showed a slight increase in spectral slope between the two Keck I/LRIS observations, the NIR showed a declining evolution between the initial NIR observation (B. Yang et al. 2025) and the Keck II/NIRES data. The average optical spectral slope of 2I/Borisov is also included (D. Jewitt & D. Z. Seligman 2023). The spectral slope of 3I/ATLAS is similar to 2I/Borisov between 0.65 and 0.9 μm.