How are blue and red litmus papers alike

They are similar in their function of checking the acidic or basic nature of a substance. Moreover, both of them exhibit only one color change; blue changes to red and back, while red changes to blue and back.

First of all, both of these papers are used to determine the acidity or pH of a certain substance. Blue litmus paper turns red in acidic solution, and retains color in basic solutions. Red litmus paper turns blue in basic solutions and retains color in acidic solutions. Both are derived from inchens.