When blue litmus paper is dipped in an acid, it turns red; when red litmus paper is dipped in an acid, it just appears to get wet. Alkacid paper, which is a universal indicator, turns orange or red when it is reacting to an acid, with a redder color indicating a lower pH and a stronger level of acidity.
It is important to use both red and blue litmus paper when testing a substance. If the substance is neutral, it just makes the red litmus paper look wet. If that same neutral substance is tested on blue litmus paper, it again makes the paper just appear wet (or a slightly darker blue) rather than turning the blue litmus paper red, as an acidic substance does.