Colors in dreams represent energy, emotions, and vibes. First consider what that single color in your dream means to you and your own personal associations and relationship with that color.
In general, pale pastel colors indicate weakness or subtlety. Dark colors represent passion and intensity. Bright colors mean awareness.
To dream in color and then dream in black and white suggests that you are starting to look at a situation from a more objective perspective instead of from an emotional standpoint. You are subconsciously reacting to events in the dream.