A PH paper is first dipped in distilled water and then in diluted solution of lemon juice. the colour of PH paper is changes from colourless.
Kindly refer to the following link:
https://www.meritnation.com/ask-answer/question/29-a-ph-paper-is-first-dipped-in-distilled-water-and-then-i/science/7867397
https://www.meritnation.com/ask-answer/question/29-a-ph-paper-is-first-dipped-in-distilled-water-and-then-i/science/7867397