While I love black & white, sometimes it is fun to bring just a little color.
There are several techniques for doing this:
For example, in the picture of the wine glasses, I wanted just the wine to retain its original color. I very carefully punched out holes for the wine. I found that I had to be really picky, or the effect was ruined. In this case, I forgot to paint back the color on the partial glass on the right edge.
When I was growing up my mother had a set of photo paints. These were oil-based semi-transparent colors that allowed you to turn black & white photos into color images (apparently because actually taking color images was too expensive). When I developed black and white in a wet dark room, I tried a similar set to bring a little color to the black & white print.
You can emulate those paints in most photo editing software. In Photoshop, for example, you create a new empty layer over your black & white image and set its mode to Color. The Color mode means that only the color information will be used from this layer. It doesn't matter how dark or light or transparent you paint on this layer, all of the luminance (light and dark) values will come from the black & white image beneath. So your colors will never muddy the final image; pure white will still be pure white no matter how storng a color you paint over it.
Obviously you wouldn't paint the whole image to emulate a color photo, as they did in my mother's time. After all a real color photo is cheaper and easier today than black and white. But if you are looking for a creative color scheme, this can be fun. In the picture of the bird I very quickly just dabbed in a little color, without worrying about staying in the lines. The final image is very different from the black & white version, or the original color version.
Lately I have been using a much faster method than either of the above. I merely tell the software to turn off the saturation on all but one or two colors. You don't have as much control. In the picture of the wine glasses I couldn't use this technique because there were other areas of red in the image and I would have lost the focus of the wine. But it is quick enough that it is fun to see what it will do to the image.
In the picture of the two girls, for example, I desaturated everything but orange. This causes most of the background to go black & white bringing the focus to the girls. It's not perfect; notice the orange bag on the back shelf and the box at their feet. But I love the effect in the final image.