  A radio "colour-colour" diagram is defined in order to determine the
evolutionary state of synchrotron-emitting plasma bubbles that are ejected
during outbursts in microquasars. We establish the colour-colour diagram for
the plasmons of SS433 observed on 18 April 1998 using VLBI observations. We
show that the radio plasmons are not consistent with a simple expanding sphere
of plasmon. This may indicate that in-situ particle acceleration is taking
place away from the central engine.
