What causes the blue-green coloration of the solution?

The blue-green coloration of a solution in the context of litmus paper turning blue when dipped into a basic solution is due to the interaction of the litmus dye with hydroxide ions (OH⁻) in the basic solution. The hydroxide ions react with the dye molecules, leading to a change in their electronic structure and, consequently, a change in the color of the dye. This transformation results in the blue-green color observed in the solution. The specific mechanism involves the acceptance of electrons by the dye from the hydroxide ions, causing the color change and indicating the presence of a base.