How to take a colour photo with black and white background with nikon d3100?

How to take a colour photo with black and white background with nikon d3100?

Don't.

This is usually best done using a program like Photoshop after you have an amazing RGB image file

There are some on-camera retouching tools, found on page 151 of the D3100 user manual.

These tools are used after you take your shots and create a new image file so you do NOT overwrite your original image file

One basic technique is to start with a color picture and then create a selection around the object to stay colored. Then invert the selection and remove the color from this inverse selection.

This is a tutorial with example for the GIMP http://aviewfinderdarkly.com.au/2012/11/15/how-to-create-a-colour-splash-photo-with-the-gimp/

the Gimp is free editing software similar to Photoshop that uses the same basic method

http://aviewfinderdarkly.com.au/2007/06/05/the-gimp-free-photo-editing-software/