An analysis is performed of multicolour observations of OJ 287 carried out during the OJ-94 Project. It is found that in the optical region the variable sources have the same power-law spectrum (F_v ∼ v^-1.5) irrespective of flux level. In the IR the spectrum flattens during the flare possibly because of synchrotron self-absorption. The comparison of the outburst of 1994 with those of 1971 and 1983 shows that there is a clear correlation between the power of outburst and colour indices of the variable component in each event: the larger is the outburst, the bluer is the variable source, i.e. in more powerful outbursts the energy spectrum of relativistic electrons is flatter.