Once you've put your intent into the spell, the color of the candle doesn't matter. As a matter of fact, the color of the candle doesn't matter at all. Your intent is what fuels the spell.
But to answer your question, if you want to stop a spell in progress, gather all the components together (blow out any candles first), place your hands over the items, and focus your intent on canceling the spell. Throw away any trash and put the salvageable pieces away.