The colour change of wood has been the topic of numerous research activities worldwide. This study investigates the colour change of Robinia (Robinia pseudoacacia L.) and two hybrid poplars, namely Pannónia poplar (Populus x euramericana cv. Pannónia) and I-214 poplar (Populus x euramericana cv. I-214). The sapwood and heartwood were investigated separately for each of the poplars. The heartwood of Robinia was also investigated. The timbers were dried in a climate chamber at four different temperatures (20 °C, 40 °C, 60 °C, and 80 °C), as the relative humidity was reduced in 5 steps (95%, 80%, 65%, 40%, and 20%) at each temperature. The colour co-ordinates L*, a*, and b* were measured according to the CIELab system. Differences in terms of colour change between wood species and sapwood versus heartwood are discussed in the paper. The effect of wood moisture content and heat on the colour co-ordinates is provided. The colour of Robinia is more sensitive to heat than poplar.