If you test a solution with red litmus paper and it turns blue, what can you conclude?

Litmus is a weakly acidic, colored organic dye. As its environment changes from acid (pH < 7) to base (pH > 7), the molecule changes from the protonated acid to the ionized salt. Its color also changes from red to blue. (The actual pH range for this color change is from about 4.5 to 8.3.) Because of this, litmus paper is widely used to distinguish between acids and bases.