Irradiation of atoms or molecules with two-color fields offers a wide range of applications such as the observation of ionization times and the control of high-harmonic generation. I will discuss mainly the observation of ionization times by using a weak probe field in addition to the strong ionizing fundamental field, either via detection of photoelectrons or via high-harmonic emission. An analytical treatment is needed to understand the underlying mechanisms and to set up equations for retrieval of information from the observables. The numerical solution of the time-dependent Schrödinger equation, on the other hand, serves as an exact reference. We find that the polarization and the frequency of the probe field are decisive parameters for setting up a useful scheme. In particular, a high-frequency orthogonal probe field (above three times the fundamental frequency) appears to be a good tool to detect the Coulomb shift of the ionization time.