  We present an efficient time domain numerical scheme for computing
spontaneous emission dynamics in colored vacua. Starting from first principles,
we map the unitary evolution of a dressed two-level quantum emitter onto the
problem of electromagnetic radiation from a complex harmonic oscillator under
self-interaction conditions. This latter oscillator-field system can be
efficiently integrated by using finite difference time domain method without
the need for calculating the photonic eigenmodes of the surrounding
environment. In contrast to earlier investigations, our computational framework
provides a unified numerical treatment for both weak and strong coupling
regimes alike. We illustrate the versatility of our scheme by considering
several different examples.
