This may appear to be a chemistry problem; but, after reading the Wikipedia article on Copper (I) Oxide, it seems to have more to do with semiconductor-physics. For example: "...light travels almost as slowly as sound in this medium.", is that true?
What have Kramers-Kronig relations got to do with it?
To a chemist, who was never brilliant at maths, it takes a bit of understanding. I know that copper (II) oxide (Mott-Hubbard insulator [semiconductor]) is black because of intervalence charge-transfer, giving rise to the generation of a highly polarising Cu(III) species; similarly, the non-stoichiometric form of nickel (II) oxide (Mott insulator) is black because of a Ni(III) species; again, silver (I) oxide is black....Ag(III) species.
This model does not appear to work for copper (I) oxide because the non-stoichiometry, causing the oxidation required for the balancing of charges with the oxide ions, would give Cu(II); which, by definition, is not sufficiently polarising to produce the deep, intense colour observed; further, the reduced Cu(I) becomes Cu(0)--the pure metal.
So, why is copper (I) oxide red?