The emitting light in beetle and fireflies arises from the electronic relaxation of oxyluciferin, an organic compound resulting from the oxidation of the D-luciferin substrate inside an enzyme called luciferase. The protein environment plays a role in the colour modulation of the light emitted. In order to theoretically study such systems [1,2], the use of quantum mechanical/molecular mechanical (QM/MM) methods is required. Accurate QM level is needed for dealing with electronic transition and charge transfer phenomena. Taking into account the surrounding protein at the MM level is essential in order to understand the colour modulation and influence of the enzyme. The presentation will discuss some of the results obtained on the bioluminescent system using the coupling of the programs MOLCAS (CASPT2/CASSCF) or G09 (DFT) and TINKER (AMBER force field). Present results based on new resolved crystallographic structures of blue-shifted coloured and red-shifted coloured species were compared to the more conventional yellow-green emitted firefly systems. The prediction and rationalisation of the influence of the mutation of some residues inside the cavity of the light emitter will be explored with theoretical studies. I will also give last results on the influence of taking into account the water environment while comparing calculated results to the experimental studies of absorption and emission of the in-solution light emitter. [1] Isabelle Navizet, Ya-Jun Liu, Nicolas Ferré, Hong-Yao Xiao, Wei-Hai Fang, Roland Lindh, Color-Tuning Mechanism of Firefly Investigated by Multi-Configurational Perturbation Method J. Am. Chem. Soc. 132, 704-712 (2010). [2] Romain Berraud-Pache and Isabelle Navizet, QM/MM calculations on a newly synthesised oxyluciferin substrate: new insights into the conformational effect, PCCP, 18, 27460 – 27467 (2016).