How to use selective color effect on the nikon d5100?

Specifically how do you choose a color before you take the photograph?

Added (1). And really? I'm asking a simple question to understand a camera better. I'm not an amateur photographer. I know what I'm doing with this camera.

It would be very wise of you to shoot the image in full color and then use photo editing software on your computer to remove the colors… And you really spent 800 bucks on a camera and you're using Selective Color? Ugh…

Why don't you simply look up… Selective Color… In the Index of your camera Manual to find the answer to your question? It's there.
But, you'd be much better off editing a copy of your images in post processing.