Ferrocene's band gap is supposed to make it colorless but it isn't. Why is it so?

Wang and Islam [[1](https://doi.org/10.48550/ARXIV.1509.05122)] state that the ferrocene molecule's band gap, i.e. the HOMO-LUMO gap at the Kohn-Sham level, is 5.03 eV, which corresponds to an absorption wavelength of at most 240 nm.


So, ferrocene should be colorless — except that it isn't. I found out that liquid water, which has an even higher band gap (7 eV) than ferrocene, is still colored [because of vibronic, and not electronic, excitations](https://en.wikipedia.org/wiki/Color_of_water).


Does the same hold for ferrocene as well, and, if it does, do the expected effects to the HOMO-LUMO gap of e.g. π-electron withdrawing/donating substituents (which would change the color of the parent molecule, were that color be dependent on the gap — again, except that it isn't) “*change*” the color in the “*expected*” ways?


P.S. Please read my long string of comments, involving explicit calculations done for the ionisation potential/negated Kohn-Sham HOMO energy of the hydride anion, on Oscar Lanzi's answer before adding more answers.


### Reference


1. Wang, F.; Islam, S. Impact of ionization of ferrocene: EOES of α- and β-electrons and the fingerprint orbital 8a₁’ of ferrocenium. **2015**. DOI: [10.48550/ARXIV.1509.05122](https://doi.org/10.48550/ARXIV.1509.05122).


