  Constraining the accretion flow geometry of black hole binaries in outburst
is complicated by the inability of simplified multi-colour disc models to
distinguish between changes in the inner disc radius and alterations to the
emergent spectrum, parameterised by the phenomenological colour correction
factor, f_col. We analyse Rossi X-ray Timing Explorer observations of the low
mass Galactic black hole X-ray binary, GX 339-4, taken over seven epochs when
the source was experiencing a state transition. The accretion disc component is
isolated using a pipeline resulting in robust detections for disc luminosities,
0.001 < L_disc / L_Edd < 0.5. Assuming that the inner disc remains situated at
the innermost stable circular orbit over the course of a state transition, we
measure the relative degree of change in f_col required to explain the spectral
evolution of the disc component. A variable f_col that increases by a factor of
~ 2.0 - 3.5 as the source transitions from the high/soft state to the low/hard
state can adequately explain the observed disc spectral evolution. For the
observations dominated by a disc component, the familiar scaling between the
disc luminosity and effective temperature, L_disc ~ T_eff^4, is observed;
however, significant deviations from this relation appear when GX 339-4 is in
the hard intermediate and low/hard states. Allowing for an evolving f_col
between spectral states, the L_disc-T_eff^4 law is recovered over the full
range of disc luminosities, although this depends heavily on the physically
conceivable range of f_col. We demonstrate that physically reasonable changes
in f_col provide a viable description for multiple state transitions of a black
hole binary without invoking radial motion of the inner accretion disc.
