  In order to accurately determine stellar properties, knowledge of the
effective temperature of stars is vital. We implement Gaia and 2MASS photometry
in the InfraRed Flux Method and apply it to over 360,000 stars across different
evolutionary stages in the GALAH DR3 survey. We derive colour-effective
temperature relations that take into account the effect of metallicity and
surface gravity over the range 4000 to 8000 kelvin, from very metal-poor stars
to super solar metallicities. The internal uncertainty of these calibrations is
of order 40-80 kelvin depending on the colour combination used. Comparison
against solar-twins, Gaia benchmark stars and the latest interferometric
measurements validates the precision and accuracy of these calibrations from F
to early M spectral types. We assess the impact of various sources of
uncertainties, including the assumed extinction law, and provide guidelines to
use our relations. Robust solar colours are also derived.
