We present calibrated Asteroid Terrestrial-impact Last Alert System (ATLAS) photometry of the interstellar comet 3I/ATLAS (C/2025 N1) from March 28 through 2025 August 29, obtained with the five-site, robotic ATLAS network in the c (420─650 nm), o (560─820 nm), and Teide w (420─720 nm) bands. Stacked difference images yield reliable light curves measured in four fixed apertures that capture the evolving coma. We observe 3I/ATLAS transitioning in color from red (c − o) ≍ 0.7 before MJD 60860 to near-solar (c − o) ≍ 0.3 after MJD 60870, coincident with the appearance of a prominent antisolar tail. The absolute magnitude curve H(t) shows a slope break near MJD 60890 at r ∼ 3.3 au from −0.035 to −0.012 mag day−1, or in terms of coma cross section as a function of heliocentric distance, r−3.9 to r−1.1. We release the aperture photometry with geometry and uncertainties to enable cross-instrument synthesis of 3I/ATLAS activity and color evolution.