Blue litmus paper turns red when grape juice is added, indicating that the solution is acidic.
As Lemon Juice is an Acid, the colour of the Red Litmus Paper will stay the same.
Potato juice is typically neutral or slightly acidic, so if you test it with litmus paper, blue litmus paper will remain blue, and red litmus paper will remain red. This indicates that potato juice does not change the color of either type of litmus paper, reflecting its neutral pH.
blue
Red
Try it out and see what happens
As orange juice being an acidic solution, it turns blue litmus into red.
It would remain red, red litmus paper only reacts to blue in the presence of a base. Lemon juice is acidic and thus wouldn't turn the paper a color. If blue litmus paper was used in the lemon juice it would turn red. Consequently dipping blue litmus paper in a base won't change the color because it does not react with bases, only acids.
Litmus is a dye extracted from lichens, a plant. Upon addition of acid, litmus turns red whilst on addition of base, litmus turns blue. Being acidic, containing citric acid, lemon juice is expected to turn violet litmus paper red.
The litmus paper will turn red when placed in orange juice, indicating that the solution is acidic.
Blue litmus paper would turn red in orange juice.
Lemon juice is an acid therefore, it turns blue litmus paper red and red litmus paper remains red