  It is now established that thermal disc emission and non-thermal jet emission
can both play a role at optical/infrared (OIR) wavelengths in X-ray transients.
The spectra of the jet and disc components differ, as do their dependence on
mass accretion properties. Here we demonstrate that the OIR colour-magnitude
diagrams (CMDs) of the evolution of the X-ray transient XTE J1550-564 in
outburst can be used to separate the disc from the jet. Monitoring in two
wavebands is all that is required. This outburst in 2000 was well studied, and
both disc and jet were known to contribute. During the outburst the data follow
a well defined path in the CMD, describing what would be expected from a heated
single-temperature blackbody of approximately constant area, except when the
data appear redder than this track. This is due to the non-thermal jet
component which dominates the OIR moreso during hard X-ray states at high
luminosities, and which is quenched in the soft state. The CMD therefore shows
state-dependent hysteresis, in analogy with (but not identical to) the well
established X-ray hardness-intensity diagram of black hole transients. The
blackbody originates in the X-ray illuminated, likely unwarped, outer accretion
disc. We show that the CMD can be approximately reproduced by a model that
assumes various correlations between X-ray, OIR disc and OIR jet fluxes. We
find evidence for the OIR jet emission to be decoupled from the disc near the
peak of the hard state.
