These are two websites to visit that will help answer the question.
Stars change color as they get older and go through their life cycle. Remember when we were talking about the H-R diagram, the stars do not stay in one place on the diagram their whole lives- they move around. Take our sun. Right now it is an ordinary G class star, but one day (about 5 billion years from now) it will die. Before that happens the sun will get larger looking for more hydrogen to burn. When it gets larger it will probably cool somewhat making it more red then yellow; remember red stars are cooler then yellow stars. When the sun cannot grow anymore it will possibly become a white dwarf. After many more years the sun will cool even more becoming a red dwarf. So, stars change color throughout their lives. We will not see a star change color in one evening, it will take thousands of years.
Stars also change color while we are looking at them because of scintillation ("Twinkling") as the light passes through the atmosphere of the Earth. As the air moves in and out, the starlight is refracted, often different colors in different directions. Because of this "chromatic aberration," stars can appear to change colors when they are twinkling strongly.