GaInN/GaN quantum dots have been grown by molecular beam epitaxy on sapphire substrates. By changing the size and composition of the dots, the emission energy can be tuned over the entire visible spectrum. We present time-resolved photoluminescence obtained on such samples with emission energies ranging from 2.4 to 3.0 eV. We observe that the radiative recombination rate of electron-hole pairs varies over several decades, in correlation with the transition energy. When the decay time of the ground-state recombination reaches several microseconds, a much faster (nanoseconds) recombination is observed at higher energy. This is tentatively explained in terms of the partial screening of the internal field by a finite number of electron-hole pairs.