Inasmuch as the fundamental electronic excitation is invariant in all privileged frames of reference by all observers, the number of delocalised  electrons in a conjugated system should in the absence of exceptions e.g. tunnelling etc., dictate the intrinsic colour of its corresponding polyene, provided that the conjugated system is not affected by other factors such as auxochromic effects, Kuhn's 15 and Woodward's rules etc. To this end, empirically six to twelve delocalised  electrons in an uncomplicated linear conjugated polyene bring about a fundamental electronic excitation with  max ~ 450 -550 nm, giving rise to coloured compounds from yellow such as n-hepta-1,3,5-triene to purplish-pink azulene (loc. cit.).