We present a comparison of experiment and theory of ultrafast one-color pump-probe multiphoton ionization spectrocopy of K-2. The wave packet propagation in the A (1)Sigma(u)(+) state and in the (2) (1)Pi(g) Rydberg state is monitored in detail by changing systematically the pump and probe wavelength from 779 nm to 837 nm. The measured total ionization rates as a function of the delay time between pump and probe are shown to depend sensitively on the pump and probe wavelengths used and exhibit drastic changes and a variety of fascinating structures as the direct observation of inward and outward wave packet detection and frequency doubling of the detected wave packet oscillation. The time dependent quantum mechanical wave packet calculations are in excellent agreement with the experimental results and allow a clear interpretation of different ionization pathways and mechanisms observed in the femtosecond ion signal.