  Photons are fundamental excitations of the electromagnetic field and can be
captured in cavities. For a given cavity with a certain size, the fundamental
mode has a fixed frequency {\it f} which gives the photons a specific "color".
The cavity also has a typical lifetime $\tau$, which results in a finite
linewidth $\delta${\it f}. If the size of the cavity is changed fast compared
to $\tau$, and so that the frequency change $\Delta${\it f} $\gg \delta${\it
f}, then it is possible to change the "color" of the captured photons. Here we
demonstrate superconducting microwave cavities, with tunable effective lengths.
The tuning is obtained by varying a Josephson inductance at one end of the
cavity. We show data on four different samples and demonstrate tuning by
several hundred linewidths in a time $\Delta t \ll \tau$. Working in the few
photon limit, we show that photons stored in the cavity at one frequency will
leak out from the cavity with the new frequency after the detuning. The
characteristics of the measured devices make them suitable for different
applications such as dynamic coupling of qubits and parametric amplification.
