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Since its market entry in 2001, ThermoWood® has become the most established form of thermal modification globally. Whilst its properties have been widely reported, there is still a need for a readily-applied quality control method to ensure treatments meet the criteria outlined by the International ThermoWood Association. One such method for quality control has focussed on the use of colour measurements. In the largest study of its kind, colour measurement data have been evaluated for Norway spruce and Scots pine subjected to the Thermo-D ThermoWood® process at twelve industrial plants between 2007 and 2018. This showed that the colour measurement according to the CIELAB* colour space on newly planed surfaces of thermally modified timber (TMT) may be used for quality control of the ThermoWood® Thermo-D process with regard to process intensity, i.e. the combined effect of temperature (212 ± 3°C) and time. In order to obtain more robust control, only the L* parameter (lightness) should be used as a quality indicator, as both a* and b* parameters for the TMT showed little variation from those of the unmodified wood and too high a scatter in values.