If a substance is very sour and has a ph of 4, then you would expect the litmus paper to turn into the shade of red. A substance that is sour and has a pH level that is below 7 would indicate that the substance is an acid or has acidic properties. For an acidic solution, the litmus would be red in color. If a blue litmus paper is used, then it would turn into red while if a red litmus paper is used, then it would remain as red. There is also a general type of litmus paper where the color change range from violet to red. A litmus is widely used in distinguishing acid and bases. It can be used in liquid solutions and in gas mixtures.