Silica-based optical fibers suffer radiation-induced attenuation (RIA) in radiation-intensive environments, severely reducing their lifespan. We propose a kinetic model based on the evolution of multiple color centers to accurately simulate defect density and quantitatively characterize RIA. The irradiation experiments were conducted using a Co-60 gamma-ray source at a dose rate of 1.25 Gy/s and a controlled temperature of 10 ± 2 °C, with measurements performed at 1310 nm and 1550 nm wavelengths. The model was experimentally validated using fluorine–germanium co-doped fibers. Fitting results demonstrate high accuracy, with a coefficient of determination (R²) > 0.999, normalized root mean square error (NRMSE) < 0.57%, and mean absolute error (MAE) < 0.74 dB/km. Furthermore, it successfully decouples complex macroscopic RIA signals into distinct transient and steady-state defect pools. This phenomenological decoupling enables band-dependent damage analysis and serves as a baseline for evaluating steady-state degradation of F-Ge co-doped fibers under constant irradiation.