As illustrated above, methanol can react with 1 to yield unwanted by-products, which perturb its colour; in effect, solutions of have a yellow tinge. As depicted in Fig. , there are a total of eight delocalised  electrons in 1 which set up a conjugated system, leading to  bonds elongation and lower energy gaps between the pertinent ungeraden and geraden molecular orbitals to such an extent that the 18 non-bonding electrons in this molecule can also participate in the delocalisation; as a the fundamental electronic excitation of 1 absorbs greenish-blue light ( max ~ 480 -490 nm), and hence this compound appears as an orange solid. Henceforth, whereas energy is variant in all privileged frames of reference by all observers, the change in energy is invariant in all privileged frames of reference for the same observers. By the same token, the fundamental electronic excitation between the pertinent ungeraden and geraden molecular orbitals of inherently coloured organic compounds is invariant in a given polyene system in any privileged frame of reference, irrespective of variant energies of different molecules upon which the conjugated system is conferred, in that privileged frame of reference by all observers. Taking as an example, albeit lycopene and β-carotene are energetically variant in a privileged frame of reference, both exhibit the same characteristic colour to all observers in that frame of reference, in virtue of their pretty much identical conjugated systems (Fig. ). It is well established that the more extended a conjugated polyene, the lower the energy gap between its HOMO u and LUMO g . Thus, as a rule of thumb, the more  electrons delocalise in a conjugated polyene system, the lower the energy gaps between its ungeraden and geraden molecular orbitals, which in turn this effect stabilises the system relative to that with equal but isolated  electrons. It follows that increasing the length of conjugation leads to a bathochromic fundamental electronic excitation. For this reason, n-hepta-1,3,5-triene with a six  electron conjugated system absorbs blue light and appears as a deep-yellow solid. 12 On the other hand, azulene with ten delocalised  electrons in its conjugated system absorbs green light, and exhibits an intrinsic purplish 13 colour.