I love to work with color which I guess comes from my fine art background. Using color to help evoke a feeling can be very rewarding when it works. Though I enjoy color, using black and white can be just as powerful. Truthfully it's really a grey scale unless you're talking about comic style inking or even hi-con film. No matter the choice they all can have a distinct visual impact even if it's not noticed by the audience. Sometimes the impact you want is for the audience to feel comfortable and to invite them in.
This painting of the houses of parliament by Monet illustrates how the use of color can change the way in which you look at a subject. It is obviously the houses of parliament in a haze but look at the palette of colors monet uses to pull you into it. This same approach can be used in film to help bring your audience in. If you're clever enough with your color you can completely manipulate your audience without them hardly realizing. These days with the ability to manipulate the color in life action films, director's are using this same technique that animation has always had the ability to do. Bambi is a great example of how color was used in animation to help drive the story and evoke certain feelings in the audience.
So when working out the color of my films I make decisions based on the story, emotion, and character and color keying is the best way to do this.