pH of lemon juice being tested using universal indicator paper (close-up at upper right). Universal indicator paper is used to measure the strength of an acid or an alkali. When a sample of the solution is added to the paper, the paper changes colour depending on the solution's pH. Yellow is neutral (pH 7), whereas orange and red are acidic, and green and dark blue are alkaline. Here, the result is orange, which is mildly acidic.