  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 f which gives the photons a specific "colour". The
cavity also has a typical lifetime tau, which results in a finite linewidth
delta f}. If the size of the cavity is changed fast compared to tau, and so
that the frequency change Delta f >> delta f, then it is possible to change the
"colour" 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 tuning by several
hundred linewidths in a time Delta t << 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 dynamic coupling of qubits.
