Make only parts of your photos black and white, with Vampix
Vampix is a free and light program that lets you convert selected colors to black and white, letting only a single color stand out in your photo.
The program provides a color picker you can use to select the color you want to keep converting all the other colors to grayscale.
Vampix provides also three sliders that let you fine tune your selection to filter a narrower or wider color range. Unfortunately the sliders won’t let you select carefully what you want, applying immediately a change even before you complete your selection.
Vampix is a program that acts as a filter. A nice idea that was not very well implemented. It is not convenient enough, but it may work find if you use a photo with a few simple / solid colors.