Answer :
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Actually Welcome to the Concept of the Litmus paper and pH indications.
Since the Litmus paper and Artificial indicators are used to check the pH of any solution.
=> If a substance is Acidic, its pH range is less than 7 , and the colour are generally red, yellow, orange and hot shades.
=> If a substance is Basic, it's pH range is more than 7 , and the colors are generally Cool shades like, blue, violet, green, etc.
==> In the lemon juice experiment, the pH paper turns from blue to vivid red, indicating the presence of H3O+ ions: lemon juice is acidic. Refer to the table of Universal Indicator Color change for clarification.