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Galling insects manipulate host plant tissues for their own benefit. Because of feeding activity of the gall insects, both structural and metabolic changes occur in the host plant, leading to the formation of an abnormal growth of new organ (the gall). Galls induced by Palaeomystella oligophaga (Lepidoptera) on Macairea radula (Melastomataceae) depend on the constant larval feeding stimulus for establishment and development. The gall consists of two tissue compartments – the storage and nutritive tissues. These two gall tissues were investigated here in terms of the levels of oxidative stress and of a possible differential chemical investment. In addition, we investigated the relationship between gall coloration and the concentration of primary metabolites. Our results supported a differential investment of chemical compounds in the M. radula compartments, with higher concentrations of polysaccharides, malondialdehyde, and phenolic compounds in the storage tissue, and higher total soluble sugar concentrations in the nutritive tissue. Regarding gall colour, conflicting results were observed in the present gall system. Water-soluble polysaccharides were detected at higher concentrations in red galls than in green galls. As a water-soluble polysaccharide, pectin seems to be related to increased anthocyanin stability, a process that might occur in galls, leading to red coloration.