Acids contribute hydrogen ions ($$\ce{H+}$$) into a solution during a chemical reaction, making them proton donors.  Bases contribute hydroxide ions ($$\ce{OH-}$$) into a solution during a chemical reaction or bond with a proton making them proton accepters.  Phenol solution turns yellow in acidic solutions and remains red in basic solutions.  There are many indicators that a chemical reaction has occurred, including color change.