Acid-base indicators. From left to right they are: methyl orange, phenolphthalein, litmus solution, blue litmus paper and red litmus paper. These change colour in the presence of acids or alkalis. Methyl orange turns red in acid and yellow in alkalis. Phenolphthalein is colourless in acids and turns pink in alkalis. Litmus is red in acids and blue in alkalis. Blue litmus paper is used to test for acids, and red for alkalis.