Two explanations might include two sets of colorless solutions that might yield a gas. What comes to mind is the production of carbon dioxide which could come from a soluble carbonate, such as sodium carbonate solution, and an acid, such as vinegar or hydrochloric acid. A second possibility is a gas such as ammonia. This could be produced by the reaction of a soluble ammonium compound, such as ammonium chloride, and a base such as sodium hydroxide.