☆Dry HCI gas does not change the colour of the litmus paper as to change the colour we need presence of ions like hydrogen and hydronium.. 
☆Moistened NH3 gas turns the red litmus paper into blue due to the ammonium chloride formed.. 
☆Lemon juice changes blue litmus to red as it contains citric acid in it.. 
☆Carbonated soft drinks have CO2 in them which forms carbonate acid and turns blue litmus to red.. 
☆Curd is a product of milk so it contains lactic acid in it and therefore it changes blue litmus to red.. 
☆Soap solution is a base and therefore changes red litmus paper to blue.. 

Hope it helps u my frnd......