While estimating the total protein through Lowry method one needs to incubate the test tubes in dark once after adding alkaline solution and once after Folin-ciocalteu solution. My teacher said it is necessary for proper colour development. But I am curious to know what is the chemistry behind it?
Sigma and this document say that the mixture is light sensitive, and it decomposed if exposed to light or stored for long periods. It's not that the reaction needs darkness to occur, just that the reagent will lose strength if the light hits it too hard, hindering the reaction with proteins and giving you less color.
Studies have not yet defined the exact mechanism of Lowry method. However, incubation in dark is suppose to promote the stability of copper tartrate complex with Folin-ciocalteu solution and thus their reaction/interaction with protein which ultimately leads to the oxidation of aromatic residues. You can read chemistry behind Lowry method for more details.