There are many philosophies and symbolism to placement and color of a front door. Take for example the teachings of Feng Shui - where the door, being the mouth of the Chi will absorb all of the nourishment for your abode. If your main door faces South, you should choose appropriate Fire element colors to attract harmonious energy, or Wood element to interplay with Fire. Red, pink, orange, green... There is also a Scottish tradition to paint your front door red when you pay off your mortgage. A red door says "welcome" to many - perhaps from the early American symbolism that a red door signified a friendly house where a weary traveler could rest their tired dogs for the night.
What does your door say about you and your home? What color is your door?