The color purple in a dream represents neutrality, or powerlessness. Something that is uncaring, unbiased, or totally powerless.
Positively, purple can symbolize having no bias, or situations that are open. This will usually be represented by lighter shades of purple.
Negatively, the color purple reflects powerlessness. You are totally unable to respond, react, or do what you want. Impotence in some form. This will usually be represented by darker shades of purple.
When you see purple in a dream it can represent a negative situation that has been "neutralized" by a positive situation, or a positive situation that has been neutralized by a negative situation.
The symbol for purple is metaphorically based on the mixture of blue and red which is waking life create purple when mixed. Since blue symbolizes positivity and red symbolizes negativity you get neutrality with purple.
Example: A young man dreamed of seeing someone wearing a purple shirt. In waking life he felt powerless to stop an enemy.
Example 2: A man dreamed of seeing a purple house. In waking life he felt completely unconcerned with someone else's problems.