why litmus paper changes color ?

Red litmus contains a weak diprotic acid. When it is exposed to a basic compound, the hydrogen ions react with the added base. The conjugated base, formed from the litmus acid, has a blue color, so the wet red litmus paper turns blue in alkaline solution.

Thank u so much .

Once you put acid, the acid will introduce an excess of H+(since it donates a proton, once it is in a water solution), bringing an excess of H+ and will shift the reaction to the left, producing more HA and turning the color red.

A corollary to that question: how does, and to what extent, does bond strength reflect the absorption of light? Bond strength being a general reference to interactions between certain bonds, in this case litmus paper. Or rather a better question is, what can describe the relationship between bond strength and light absorption?