We present calibrated ATLAS photometry of the interstellar comet 3I/ATLAS (C/2025 N1) from 28 March through 29 Aug 2025, 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 p…