Relatively intense deep-green/yellow photoluminescence emission at ∼600 nm is observed for InGaN/GaN multi quantum well (MQW) structures grown on bulk AlN substrates, demonstrating the potential to extend commercial III-Nitride LED technology to longer wavelengths. Optical spectroscopy has been performed on InGaN MQWs with an estimated In concentration of greater than 50% grown by metalorganic chemical vapor phase epitaxy at 750°C. Temperature- and power-dependence, time-resolved photoluminescence as well as spatially resolved cathodo-luminescence measurements and transmission electron microscopy have been applied to understand and elucidate the nature of the mechanism responsible for radiative recombination at 600nm as well as higher energy emission band observed in the samples. A comparison between samples grown on bulk AlN and sapphire substrates indicate a lower degree of compositional and/or thickness fluctuation in the latter case. Our results indicate the presence of alloy compositional fluctuation in the active region despite the lower strain expected in the structure contrary to that of low In composition active regions deposited on bulk GaN substrates. Transient photoluminescence measurements signify a stretched exponential followed by a power decay to best fit the luminescence decay indicative of carrier hopping in the active region. Our results point to the fact that at such high In composition (<30%) InGaN compositional fluctuation is still a dominant effect despite lower strain at the substrate-epi interface.